Change help text for --pool-size parameter
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.

ArtemZ/wal-e,nagual13/wal-e,wal-e/wal-e,x86Labs/wal-e,intoximeters/wal-e,alex/wal-e,RichardKnop/wal-e,heroku/wal-e,equa/wal-e,modulexcite/wal-e,ajmarks/wal-e,tenstartups/wal-e,DataDog/wal-e,fdr/wal-e