Doc: mention that VACUUM can't utilize over 1GB of RAM

Document that setting maintenance_work_mem to values over 1GB has no
effect on VACUUM.

Reported-by: Martín Marqués
Author: Laurenz Albe
Discussion: https://postgr.es/m/CABeG9LsZ2ozUMcqtqWu_-GiFKB17ih3p8wBHXcpfnHqhCnsc7A%40mail.gmail.com
Backpatch-through: 9.6, oldest supported release

doc/src/sgml/config.sgml

@@ -1893,6 +1893,12 @@ include_dir 'conf.d'
         too high.  It may be useful to control for this by separately
         setting <xref linkend="guc-autovacuum-work-mem"/>.
        </para>
+       <para>
+        Additionally, <command>VACUUM</command> is only able to utilize up to
+        a maximum of <literal>1GB</literal> of memory, so
+        <varname>maintenance_work_mem</varname> values higher than this have
+        no effect on <command>VACUUM</command>.
+       </para>
       </listitem>
      </varlistentry>