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