doc: Clarify sendrawtransaction::maxfeerate==0 help

Summary: This is a backport of Core PR15760

Test Plan:
ninja
src/bitcoin-cli help sendrawtransaction

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8113