doc: Change documentation for =0 for non-boolean option

Summary:

PR #12713 (D1833) changed the interpretation for negation of non-boolean options (e.g. -noconnect) to no longer set the option to 0, but to remove it from the options.

I think this is better because it gets rid of the special meaning of '0'.

However it needs to be documented. I attempt to do so in this PR.

This is a backport of core#14100
This backport was accidentally started in D12561, all the args in common.cpp have been taken care of.

Test Plan: proof-read

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D19736