doc: Improve consistency of parallel vacuum description.
Use consistent phrasing for parallel vacuum descriptions between
manual VACUUM and autovacuum. Specifically, clarify that the parallel
worker count is limited by the respective options only if they are
explicitly specified.
Also, fix a typo in the parallel vacuum section.
Author: Aleksander Alekseev <aleksander@tigerdata.com>
Discussion: https://postgr.es/m/CAJ7c6TPcSqzhbhrsiCMmVwmE8F7pwS7i9J49SP1zPKS_ER+vcA@mail.gmail.com