doc: Mention pg_read_all_stats in description of track_activities

The description of track_activities mentioned that it is visible to
superusers and that the information related to the current session can
be seen, without telling about pg_read_all_stats.  Roles that are
granted the privileges of pg_read_all_stats can also see this
information, so mention it in the docs.

Author: Ian Barwick
Reviewed-by: Nathan Bossart
Discussion: https://postgr.es/m/CAB8KJ=jhPyYFu-A5r-ZGP+Ax715mUKsMxAGcEQ9Cx_mBAmrPow@mail.gmail.com
Backpatch-through: 10

Author: michaelpq

doc/src/sgml/config.sgml

@@ -7916,10 +7916,10 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
         Enables the collection of information on the currently
         executing command of each session, along with its identifier and the
         time when that command began execution. This parameter is on by
-        default. Note that even when enabled, this information is not
-        visible to all users, only to superusers and the user owning
-        the session being reported on, so it should not represent a
-        security risk.
+        default. Note that even when enabled, this information is only
+        visible to superusers, members of the
+        <literal>pg_read_all_stats</literal> role and the user owning the
+        session being reported on, so it should not represent a security risk.
         Only superusers and users with the appropriate <literal>SET</literal>
         privilege can change this setting.
        </para>