Backport PR11309: Minor cleanups for AcceptToMemoryPool

Summary:
This is based on #11309 https://github.com/bitcoin/bitcoin/pull/11309/files however
the removal of fLimitFree and the fix for reorgs was already done in D4745.
So, this is basically just renaming fOverrideMempoolLimit -> bypass_limits,
removing defaultness of two args, and making sure that literal arguments are
documented with /* */. (test_accept from D3079 stays as default arg, like it
is in Core today)

Test Plan: ninja check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4796