Revamp documentation for predefined roles.

Presently, the page for predefined roles contains a table with
brief descriptions of what each role allows.  Below the table,
there is a separate section with more detailed information about
some of the roles.  As the set of predefined roles has grown over
the years, this page has (IMHO) become less readable.

This commit attempts to improve the predefined roles documentation
by abandoning the table in favor of listing each role with its own
complete description, similar to how we document GUCs.  Besides
merging the information that was split between the table and the
section below it, this commit also alphabetizes the roles.  The
alphabetization is imperfect because some of the roles are grouped
(e.g., pg_read_all_data and pg_write_all_data), and we order such
groups by the first role mentioned, but that seemed like a better
choice than breaking the groups apart.  Finally, this commit makes
some stylistic adjustments to the text.

Reviewed-by: David G. Johnston, Robert Haas
Discussion: https://postgr.es/m/ZmtM-4-eRtq8DRf6%40nathan

doc/src/sgml/monitoring.sgml

@@ -286,8 +286,8 @@ postgres   27093  0.0  0.0  30096  2752 ?        Ss   11:34   0:00 postgres: ser
    other sessions, many columns will be null.  Note, however, that the
    existence of a session and its general properties such as its sessions user
    and database are visible to all users.  Superusers and roles with privileges of
-   built-in role <literal>pg_read_all_stats</literal> (see also <xref
-   linkend="predefined-roles"/>) can see all the information about all sessions.
+   built-in role <link linkend="predefined-role-pg-monitor"><literal>pg_read_all_stats</literal></link>
+   can see all the information about all sessions.
   </para>
 
   <table id="monitoring-stats-dynamic-views-table">