rpc: Add doc for deactivating settxfee and respect -maxtxfee wallet setting

Summary:
PR description:

settxfee can be deactivated by passing 0 as the fee. That does not seem to be documented for the user so this PR adds it in the description.
Added a commit that makes settxfee respect the setting -maxtxfee

This is a backport of Core PR18467

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8939