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

doc/src/sgml/catalogs.sgml

@@ -1893,8 +1893,8 @@ SCRAM-SHA-256$<replaceable>&lt;iteration count&gt;</replaceable>:<replaceable>&l
   </indexterm>
 
   <para>
-   The catalog <structname>pg_class</structname> catalogs tables and most
-   everything else that has columns or is otherwise similar to a
+   The catalog <structname>pg_class</structname> describes tables and
+   other objects that have columns or are otherwise similar to a
    table.  This includes indexes (but see also <link
    linkend="catalog-pg-index"><structname>pg_index</structname></link>),
    sequences (but see also <link
@@ -1902,8 +1902,8 @@ SCRAM-SHA-256$<replaceable>&lt;iteration count&gt;</replaceable>:<replaceable>&l
    views, materialized views, composite types, and TOAST tables;
    see <structfield>relkind</structfield>.
    Below, when we mean all of these kinds of objects we speak of
-   <quote>relations</quote>.  Not all columns are meaningful for all relation
-   types.
+   <quote>relations</quote>.  Not all of <structname>pg_class</structname>'s
+   columns are meaningful for all relation kinds.
   </para>
 
   <table>