doc: Add details about object "wal" in pg_stat_io

This commit adds a short description of what kind of activity is tracked
in pg_stat_io for the object "wal", with a link pointing to the section
"WAL configuration" that has a lot of details on the matter.

This should perhaps have been added in a051e71e28, but things are what
they are.

Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Discussion: https://postgr.es/m/Z7RkQ0EfYaqqjgz/@ip-10-97-1-34.eu-west-3.compute.internal

doc/src/sgml/monitoring.sgml

@@ -2930,6 +2930,16 @@ description | Waiting for a newly initialized WAL file to reach durable storage
    writer</literal>.
   </para>
 
+  <para>
+   For the <varname>object</varname> <literal>wal</literal>,
+   <varname>fsyncs</varname> and <varname>fsync_time</varname> track the
+   fsync activity of WAL files done in <function>issue_xlog_fsync</function>.
+   <varname>writes</varname> and <varname>write_time</varname>
+   track the write activity of WAL files done in
+   <function>XLogWrite</function>.
+   See <xref linkend="wal-configuration"/> for more information.
+  </para>
+
   <para>
    <structname>pg_stat_io</structname> can be used to inform database tuning.
    For example: