PR rationale:
> This was a requested follow-up to #19133 and #17700 to alleviate confusion and head-scratching by people following tutorials that use `generate`.
> test: consider generate covered in _get_uncovered_rpc_commands()
> rpc: print useful help and error message for generate
> test: add rpc_generate functional test
This is a backport of [[ https://github.com/bitcoin/bitcoin/pull/19455 | core#19455 ]]