Clarify optime creation in TransactionParticipant

There is a line here that is no longer 100% true:

// Transactions do not survive term changes, so combining "getTerm" here with the

// recovery unit timestamp does not cause races.

_speculativeTransactionReadOpTime = {readTimestamp, replCoord->getTerm()};

This is no longer true for prepared transactions since they can survive term changes. I expect this is still safe since prepared transactions will do a write and not wait on the _speculativeTransactionReadOpTime but this should be double checked, and commented as such.

"OpTime creation" in the transaction participant is done here as well, very similarly.