Improve the description of various GUCs

Improve the description of various GUCs

This commit fixes a couple of inconsistencies in the descriptions of
some GUCs, while making their wording more general regarding the units
they rely on.

For most of them, this removes the use of terms like "N seconds" or "N
bytes", which may not apply easily to all the languages these strings
are translated to (from my own experience, this works in French and
English, less in Japanese).

Per debate between the authors listed below.

Author: Justin Pryzby, Michael Paquier
Discussion: https://postgr.es/m/20211129030833.GJ17618@telsasoft.com