Merge #15746: rpc: RPCHelpMan: Always name dictionary keys

Summary:
fa26eb5e8f rpc: RPCHelpMan: Always push_name when outer type is an object (MarcoFalke)
fa652b229e rpc: Add some doxygen comments to utils (MarcoFalke)

Pull request description:

Fixes two issues reported in #15737:

* > I am very perplexed as to how the code I'm looking at is generating the help text I'm seeing

So add documentation

* > This is a value for which a key is missing

So always serialize the name of the dictionary key if the outer type is a dictionary

ACKs for commit fa26eb:

promag:
  Tested ACK fa26eb5.

Tree-SHA512: b6f0cee1f1123d245d4902e8e113b5260cae7f2cb39c9bfb8893c5b0b33ffb6349ad05813d560d39a94ccf655399c05fcda15d9b0733e6bd696538fe0aca7021

Backport of Core PR15746

Test Plan:
On master:

ninja
./bitcoind
./bitcoin-cli help > RPCs
./getrpchelps.sh RPCs master

Then on the patched branch:

ninja
./bitcoind
./getrpchelps.sh RPCs patched
./checkrpc.sh RPCs master patched help.diff

Verify changes in help.diff.

Scripts:

help.diff:

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6231