doc: Update wording about direct system catalog manipulation

It was previously suggested that "esoteric" operations such as creating
a new access method would require direct manipulation of the system
catalogs, but that example has gone away, and I can't think of a new one
to replace it, so just put in some weasel wording.

Author: petere

doc/src/sgml/catalogs.sgml

@@ -14,11 +14,13 @@
    tables.  You can drop and recreate the tables, add columns, insert
    and update values, and severely mess up your system that way.
    Normally, one should not change the system catalogs by hand, there
-   are always SQL commands to do that.  (For example, <command>CREATE
+   are normally SQL commands to do that.  (For example, <command>CREATE
    DATABASE</command> inserts a row into the
    <structname>pg_database</structname> catalog &mdash; and actually
    creates the database on disk.)  There are some exceptions for
-   particularly esoteric operations, such as adding index access methods.
+   particularly esoteric operations, but many of those have been made
+   available as SQL commands over time, and so the need for direct manipulation
+   of the system catalogs is ever decreasing.
   </para>
 
  <sect1 id="catalogs-overview">