Add guide to MEV logs (#3611)

## Proposed Changes

Add some docs on checking the builder configuration, which is a frequently asked question on Discord.

## Additional Info

My text editor also insisted on stripping some trailing newlines, but can put 'em back if we want

book/src/builders.md

@@ -45,24 +45,24 @@ relays, run one of the following services and configure lighthouse to use it wit
 
 ## Validator Client Configuration
 
-In the validator client you can configure gas limit and fee recipient on a per-validator basis. If no gas limit is 
+In the validator client you can configure gas limit and fee recipient on a per-validator basis. If no gas limit is
-configured, Lighthouse will use a default gas limit of 30,000,000, which is the current default value used in execution 
+configured, Lighthouse will use a default gas limit of 30,000,000, which is the current default value used in execution
-engines.  You can also enable or disable use of external builders on a per-validator basis rather than using 
+engines.  You can also enable or disable use of external builders on a per-validator basis rather than using
 `--builder-proposals`, which enables external builders for all validators. In order to manage these configurations
 per-validator, you can either make updates to the `validator_definitions.yml` file or you can use the HTTP requests
 described below.
 
 Both the gas limit and fee recipient will be passed along as suggestions to connected builders. If there is a discrepancy
 in either, it will *not* keep you from proposing a block with the builder. This is because the bounds on gas limit are
-calculated based on prior execution blocks, so an honest external builder will make sure that even if your 
+calculated based on prior execution blocks, so an honest external builder will make sure that even if your
-requested gas limit value is out of the specified range, a valid gas limit in the direction of your request will be 
+requested gas limit value is out of the specified range, a valid gas limit in the direction of your request will be
 used in constructing the block. Depending on the connected relay, payment to the proposer might be in the form of a
-transaction within the block to the fee recipient, so a discrepancy in fee recipient might not indicate that there 
+transaction within the block to the fee recipient, so a discrepancy in fee recipient might not indicate that there
-is something afoot. 
+is something afoot.
 
-> Note: The gas limit configured here is effectively a vote on block size, so the configuration should not be taken lightly. 
+> Note: The gas limit configured here is effectively a vote on block size, so the configuration should not be taken lightly.
 > 30,000,000 is currently seen as a value balancing block size with how expensive it is for
-> the network to validate blocks. So if you don't feel comfortable making an informed "vote", using the default value is 
+> the network to validate blocks. So if you don't feel comfortable making an informed "vote", using the default value is
 > encouraged. We will update the default value if the community reaches a rough consensus on a new value.
 
 ### Set Gas Limit via HTTP
@@ -157,20 +157,71 @@ By default, Lighthouse is strict with these conditions, but we encourage users t
 - `--builder-fallback-disable-checks` - This flag disables all checks related to chain health. This means the builder
   API will always be used for payload construction, regardless of recent chain conditions.
 
-## Builder Profit Threshold 
+## Builder Profit Threshold
 
 If you are generally uneasy with the risks associated with outsourced payload production (liveness/censorship) but would
 consider using it for the chance of out-sized rewards, this flag may be useful:
 
 `--builder-profit-threshold <WEI_VALUE>`
 
-The number provided indicates the minimum reward that an external payload must provide the proposer for it to be considered 
+The number provided indicates the minimum reward that an external payload must provide the proposer for it to be considered
 for inclusion in a proposal. For example, if you'd only like to use an external payload for a reward of >= 0.25 ETH, you
-would provide your beacon node with `--builder-profit-threshold 250000000000000000`. If it's your turn to propose and the 
+would provide your beacon node with `--builder-profit-threshold 250000000000000000`. If it's your turn to propose and the
 most valuable payload offered by builders is only 0.1 ETH, the local execution engine's payload will be used. Currently,
-this threshold just looks at the value of the external payload. No comparison to the local payload is made, although 
+this threshold just looks at the value of the external payload. No comparison to the local payload is made, although
 this feature will likely be added in the future.
 
+## Checking your builder config
+
+You can check that your builder is configured correctly by looking for these log messages.
+
+On start-up, the beacon node will log if a builder is configured:
+
+```
+INFO Connected to external block builder
+```
+
+At regular intervals the validator client will log that it successfully registered its validators
+with the builder network:
+
+```
+INFO Published validator registrations to the builder network
+```
+
+When you succesfully propose a block using a builder, you will see this log on the beacon node:
+
+```
+INFO Successfully published a block to the builder network
+```
+
+If you don't see that message around the time of your proposals, check your beacon node logs
+for `INFO` and `WARN` messages indicating why the builder was not used.
+
+Examples of messages indicating fallback to a locally produced block are:
+
+```
+INFO No payload provided by connected builder.
+```
+
+```
+WARN Unable to retrieve a payload from a connected builder
+```
+
+```
+INFO The value offered by the connected builder does not meet the configured profit threshold.
+```
+
+```
+INFO Due to poor chain health the local execution engine will be used for payload construction.
+```
+
+In case of fallback you should see a log indicating that the locally produced payload was
+used in place of one from the builder:
+
+```
+INFO Reconstructing a full block using a local payload
+```
+
 [mev-rs]: https://github.com/ralexstokes/mev-rs
 [mev-boost]: https://github.com/flashbots/mev-boost
 [gas-limit-api]: https://ethereum.github.io/keymanager-APIs/#/Gas%20Limit