doc: add missing "testnet4" network string in RPC/init help texts
requested to merge github/fork/theStack/202408-doc-add_missing_testnet4_in_rpc/init_help into master
The following bitcoind parameters / RPC calls still missed the "testnet4" network string:
- -chain= parameter
- getblockchaininfo RPC, "chain" result
- getmininginfo RPC, "chain" result
The occurences were found via $ git grep \".*main.*test.*\".