Fix docs about backend functions that lock itransfer->lock

In 2013, commit 858b794c added a comment saying "This function gets
called with the flying_transfers_lock locked".

That appears to have changed with 138b661f. Commit 88778414 improved
some stale comments, but missed these.

It is clear in the code that the comment is no longer true. The function
is *not* called with flying_transfers_lock locked, but it does lock
itransfer->lock.

References #1410
2 files changed