Doc: copy-edit the introductory para for the pg_class catalog.

Doc: copy-edit the introductory para for the pg_class catalog.

The previous wording had a faint archaic whiff to it, and more
importantly used "catalogs" as a verb, which while cutely
self-referential seems likely to provoke confusion in this
particular context.  Also consistently use "kind" not "type" to
refer to the different kinds of relations distinguished by relkind.

Per gripe from Martin Nash.  Back-patch to supported versions.

Discussion: https://postgr.es/m/169518739902.3727338.4793815593763320945@wrigleys.postgresql.org