docs: add block retention to upgrading.md (#5284)

## Description

Add block pruning information to `upgrading.md`
Closes: #XXX

UPGRADING.md

@@ -156,6 +156,19 @@ is recommended to check the chainID if using the same key management system for
 When upgrading to version 0.33.4 you will have to fetch the `third_party`
 directory along with the updated proto files.
 
+### Block Retention
+
+ResponseCommit added a field for block retention. The application can provide information to Tendermint on how to prune blocks.
+If an application would like to not prune any blocks pass a `0` in this field.
+
+```proto
+message ResponseCommit {
+  // reserve 1
+  bytes  data          = 2; // the Merkle root hash
+  ++ uint64 retain_height = 3; // the oldest block height to retain ++
+}
+```
+
 ## v0.33.0
 
 This release is not compatible with previous blockchains due to commit becoming