v0.31.6 changelog (#3688)

* update changelog

* Update changelog

- less detail about internal stuff like the `mempool`
- focus BUG FIX entries more on what was fixed rather than how

* minor cleanup.

- remove entry for GenesisDocProvider, it's being undone

* minor fix

types/block.go

@@ -584,6 +584,7 @@ func (commit *Commit) VoteSignBytes(chainID string, valIdx int) []byte {
 
 // memoizeHeightRound memoizes the height and round of the commit using
 // the first non-nil vote.
+// Should be called before any attempt to access `commit.height` or `commit.round`.
 func (commit *Commit) memoizeHeightRound() {
 	if len(commit.Precommits) == 0 {
 		return