Improve banman behavior comments

Summary:
See https://github.com/bitcoin/bitcoin/pull/19219/commits/b691f2df5f7d443c0c9ee056ab94aa0fc19566d5#diff-62ade503a98fc4b68580dc91372eb491
The new comment was missed since we backported this commit before it was merged upstream.
The removed comment is not present in the merged version of the commit, but was present
when we backported (see D6506).

Test Plan: Read it.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6868