This fully comments all the TxRelay members. The only significant change
is to the comment for m_relay_txs. Previously the comment stated that
one of the purposes of the field was that "We don't relay tx invs before
receiving the peer's version message". However, even without the
m_relay_txs flag, we would not send transactions to the peer before
receiving the `version` message, since SendMessages() returns
immediately if fSuccessfullyConnected is not set to true, which only
happens once a `version` and `verack` message have been received.
This is a partial backport of core#22778
https://github.com/bitcoin/bitcoin/pull/22778/commits/290a8dab0288344fa5731ec2ffd09478e9420a2f
Depends on D14738