Ensure two newlines in help text between RPC command and description

Summary:
A few commands are missing this newline. Instead of fixing those directly,
it's best to ensure there's always two newlines between the command name/signature
and description, as it reduces the amount of location-aware formatting when
constructing RPCHelpMan objects.

While it may be possible to lint this for future changes, the long-term impact of such
a linter is not necessary positive since RPCHelpMan and related code are still undergoing
substantial refactors. We can reconsider as the code matures.

Test Plan:

ninja check check-functional

./bitcoind -regtest
(set -e ; ./bitcoin-cli -regtest help | grep -v "==" | awk NF | cut -d " " -f1 | xargs -n1 bash -c './bitcoin-cli -regtest help $0 | head -n3 && echo ---' )

Verify that all help text starts with:

Command name/signature

Description...

The only one that does not match this is pruneblockchain which is missing a description all together.

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D7184