clarify RPC rawtransaction documentation

Summary:
This is a backport of Core PR14808

One of the changes (privkyes -> privkeys) was already fixed in D5720

Test Plan:
ninja

src/bitcoin-cli help testmempoolaccept

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8112