Add comment explaining intended use of m_tx_relay

Summary: This is a partial backport of Core PR15759 : https://github.com/bitcoin/bitcoin/pull/15759/commits/b83f51a4bbe29bf130a2b0c0e85e5bffea107f75

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D6428