Document units for max_slot_wal_keep_size

The doc blurb failed to mention units, as well as lacking the point
about changeability.

Backpatch to 13.
Reviewed-by: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Reported by: b1000101@pm.me
Discussion: https://postgr.es/m/163760291192.26193.10801700492025355788@wrigleys.postgresql.org

doc/src/sgml/config.sgml

@@ -4171,6 +4171,9 @@ restore_command = 'copy "C:\\server\\archivedir\\%f" "%p"'  # Windows
         to continue replication due to removal of required WAL files. You
         can see the WAL availability of replication slots
         in <link linkend="view-pg-replication-slots">pg_replication_slots</link>.
+        If this value is specified without units, it is taken as megabytes.
+        This parameter can only be set in the <filename>postgresql.conf</filename>
+        file or on the server command line.
        </para>
        </listitem>
       </varlistentry>