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

doc/src/sgml/config.sgml

@@ -7522,7 +7522,8 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
         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
+        visible to all users, only 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 can change this setting.