docs: authz client spec (#10136)

## Description

Ref: #9707

<!-- Add a description of the changes that this PR introduces and the files that
are the most critical to review. -->

- Add `client.md` to module specification.
- Add `client.md` page to `authz` module spec.

---

### Author Checklist

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please add links to any relevant follow up issues.*

I have...

- [x] included the correct `docs:` prefix in the PR title
- [x] targeted the correct branch (see [PR Targeting](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#pr-targeting))
- [x] provided a link to the relevant issue or specification
- [x] followed the [documentation writing guidelines](https://github.com/cosmos/cosmos-sdk/blob/master/docs/DOC_WRITING_GUIDELINES.md)
- [x] reviewed "Files changed" and left comments if necessary
- [ ] confirmed all CI checks have passed

### Reviewers Checklist

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I have...

- [ ] confirmed the correct `docs:` prefix in the PR title
- [ ] confirmed all author checklist items have been addressed 
- [ ] confirmed that this PR only changes documentation
- [ ] reviewed content for consistency
- [ ] reviewed content for thoroughness
- [ ] reviewed content for spelling and grammar
- [ ] tested instructions (if applicable)

docs/spec/SPEC-SPEC.md

@@ -34,6 +34,7 @@ following list is nonbinding and all files are optional.
 - `XX_end_block.md` - specify any end-block operations
 - `XX_hooks.md` - describe available hooks to be called by/from this module
 - `XX_events.md` - list and describe event tags used
+- `XX_client.md` - list and describe CLI commands and gRPC and REST endpoints
 - `XX_params.md` - list all module parameters, their types (in JSON) and examples
 - `XX_future_improvements.md` - describe future improvements of this module
 - `XX_appendix.md` - supplementary details referenced elsewhere within the spec