Merge PR #3717: Allow the undelegation of more coins than were delegated; More validity checks.

docs/spec/auth/05_vesting.md

@@ -251,10 +251,12 @@ func DelegateCoins(t Time, from Account, amount Coins) {
 ### Undelegating
 
 For a vesting account attempting to undelegate `D` coins, the following is performed:
+NOTE: `DV < D` and `(DV + DF) < D` may be possible due to quirks in the rounding of
+delegation/undelegation logic.
 
-1. Verify `(DV + DF) >= D > 0` (this is simply a sanity check)
+1. Verify `D > 0`
 2. Compute `X := min(DF, D)` (portion of `D` that should become free, prioritizing free coins)
-3. Compute `Y := D - X` (portion of `D` that should remain vesting)
+3. Compute `Y := min(DV, D - X)` (portion of `D` that should remain vesting)
 4. Set `DF -= X`
 5. Set `DV -= Y`
 6. Set `BC += D`
@@ -274,6 +276,11 @@ func (cva ContinuousVestingAccount) TrackUndelegation(amount Coins) {
 with an excess `DV` amount, even after all its coins have vested. This is because
 undelegating free coins are prioritized.
 
+**Note**: The undelegation (bond refund) amount may exceed the delegated
+vesting (bond) amount due to the way undelegation truncates the bond refund,
+which can increase the validator's exchange rate (tokens/shares) slightly if the
+undelegated tokens are non-integral.
+
 #### Keepers/Handlers
 
 ```go