Improve consistency of comments in system catalog headers.

Improve consistency of comments in system catalog headers.

Use the term "system catalog" rather than "system relation" in assorted
places where it's clearly referring to a table rather than, say, an
index.  Use more natural word order in the header boilerplate, improve
some of the one-liner catalog descriptions, and fix assorted random
deviations from the normal boilerplate.  All purely neatnik-ism, but
why not.

John Naylor, some additional cleanup by me

Discussion: https://postgr.es/m/CAJVSVGUeJmFB3h-NJ18P32NPa+kzC165nm7GSoGHfPaN80Wxcw@mail.gmail.com