docs: re-add priv val state munging for upgrades

We recently removed mention of zeroing-out `priv_validator_state.json`
for CometBFT as part of the chain upgrade docs. During migration testing
on a private fork today, though, several of us observed that without
this change, the network would not resume. More research required to
understand the root cause here.

Refs #4245, #4239.

docs/guide/src/node/pd/chain-upgrade.md

@@ -65,7 +65,8 @@ and provide that value in the documentation. Or should we just pick a genesis st
    You must use that precise genesis time, otherwise your node will not be able to reach consensus with the rest of the network.
 6. Move the migrated state into place: `rm -r ~/.penumbra/testnet_data/node0/pd/rocksdb && mv ~/.penumbra/testnet_data/node0/pd-exported-state/rocksdb ~/.penumbra/testnet_data/node0/pd/`
 7. Copy the new genesis into place: `cp ~/.penumbra/testnet_data/node0/pd-exported-state/genesis.json ~/.penumbra/testnet_data/node0/cometbft/config/genesis.json`
-8. Clean up the old CometBFT state: `find ~/.penumbra/testnet_data/node0/cometbft/data/ -mindepth 1 -maxdepth 1 -type d -and -not -name tx_index.db -exec rm -r {} +`
+8. Copy the new signing state into place: `cp ~/.penumbra/testnet_data/node0/pd-exported-state/priv_validator_state.json ~/.penumbra/testnet_data/node0/cometbft/data/priv_validator_state.json`
+9. Clean up the old CometBFT state: `find ~/.penumbra/testnet_data/node0/cometbft/data/ -mindepth 1 -maxdepth 1 -type d -and -not -name tx_index.db -exec rm -r {} +`
 
 <!--
 N.B. We use an ugly ad-hoc find command rather than `cometbft reset-state` because we want to preserve the `tx_index.db` directory.