Doc: improve documentation of pg_signal_backend default role.

Give it an explanatory para like the other default roles have.
Don't imply that it can send any signal whatever.

In passing, reorder the table entries and explanatory paras
for the default roles into some semblance of consistency.

Ian Barwick, tweaked a bit by me.

Discussion: https://postgr.es/m/89907e32-76f3-7282-a89c-ea19c722fe5d@2ndquadrant.com

doc/src/sgml/user-manag.sgml

@@ -530,9 +530,16 @@ DROP ROLE doomed_role;
        <entry>Execute monitoring functions that may take <literal>ACCESS SHARE</literal> locks on tables,
        potentially for a long time.</entry>
       </row>
+      <row>
+       <entry>pg_monitor</entry>
+       <entry>Read/execute various monitoring views and functions.
+       This role is a member of <literal>pg_read_all_settings</literal>,
+       <literal>pg_read_all_stats</literal> and
+       <literal>pg_stat_scan_tables</literal>.</entry>
+      </row>
       <row>
        <entry>pg_signal_backend</entry>
-       <entry>Send signals to other backends (eg: cancel query, terminate).</entry>
+       <entry>Signal another backend to cancel a query or terminate its session.</entry>
       </row>
       <row>
        <entry>pg_read_server_files</entry>
@@ -549,27 +556,10 @@ DROP ROLE doomed_role;
        <entry>Allow executing programs on the database server as the user the database runs as with
        COPY and other functions which allow executing a server-side program.</entry>
       </row>
-      <row>
-       <entry>pg_monitor</entry>
-       <entry>Read/execute various monitoring views and functions.
-       This role is a member of <literal>pg_read_all_settings</literal>,
-       <literal>pg_read_all_stats</literal> and
-       <literal>pg_stat_scan_tables</literal>.</entry>
-      </row>
      </tbody>
     </tgroup>
    </table>
 
-  <para>
-  The <literal>pg_read_server_files</literal>, <literal>pg_write_server_files</literal> and
-  <literal>pg_execute_server_program</literal> roles are intended to allow administrators to have
-  trusted, but non-superuser, roles which are able to access files and run programs on the
-  database server as the user the database runs as.  As these roles are able to access any file on
-  the server file system, they bypass all database-level permission checks when accessing files
-  directly and they could be used to gain superuser-level access, therefore care should be taken
-  when granting these roles to users.
-  </para>
-
   <para>
   The <literal>pg_monitor</literal>, <literal>pg_read_all_settings</literal>,
   <literal>pg_read_all_stats</literal> and <literal>pg_stat_scan_tables</literal>
@@ -579,6 +569,25 @@ DROP ROLE doomed_role;
   other system information normally restricted to superusers.
   </para>
 
+  <para>
+  The <literal>pg_signal_backend</literal> role is intended to allow
+  administrators to enable trusted, but non-superuser, roles to send signals
+  to other backends. Currently this role enables sending of signals for
+  canceling a query on another backend or terminating its session. A user
+  granted this role cannot however send signals to a backend owned by a
+  superuser.  See <xref linkend="functions-admin-signal"/>.
+  </para>
+
+  <para>
+  The <literal>pg_read_server_files</literal>, <literal>pg_write_server_files</literal> and
+  <literal>pg_execute_server_program</literal> roles are intended to allow administrators to have
+  trusted, but non-superuser, roles which are able to access files and run programs on the
+  database server as the user the database runs as.  As these roles are able to access any file on
+  the server file system, they bypass all database-level permission checks when accessing files
+  directly and they could be used to gain superuser-level access, therefore
+  great care should be taken when granting these roles to users.
+  </para>
+
   <para>
   Care should be taken when granting these roles to ensure they are only used where
   needed and with the understanding that these roles grant access to privileged
@@ -586,8 +595,8 @@ DROP ROLE doomed_role;
   </para>
 
   <para>
-   Administrators can grant access to these roles to users using the GRANT
-   command:
+   Administrators can grant access to these roles to users using the
+   <xref linkend="sql-grant"/> command, for example:
 
 <programlisting>
 GRANT pg_signal_backend TO admin_user;