Amend both code and comment in cmd.py for wal-push/wal-fetch

In addition to having been an outright falsehood (wal-push and
wal-fetch comments were swapped) the wal-fetch operator was
confusingly spread out across the body of the function.
Change help text for --pool-size parameter

This referred specifically to downloads, but as a convenience of
mechanism the exact same setting is used for uploads, also.

Instead of teaching everything two help texts, unify, as it seems
clear enough.
Draft patch