doc: Fix XML_CATALOG_FILES env var for Apple M1 machines

Homebrew changed the prefix for Apple M1 based machines, so our
advice for XML_CATALOG_FILES needs to mention both.  More info
on the Homebrew change can be found at:

  https://github.com/Homebrew/brew/issues/9177

Author: Julien Rouhaud <julien.rouhaud@free.fr>
Discussion: https://postgr.es/m/20230327082441.h7pa2vqiobbyo7rd@jrouhaud

doc/src/sgml/docguide.sgml

@@ -209,9 +209,13 @@ brew install docbook docbook-xsl libxslt fop
 
    <para>
     The Homebrew-supplied programs require the following environment variable
-    to be set:
+    to be set.  For Intel based machines, use this:
 <programlisting>
 export XML_CATALOG_FILES=/usr/local/etc/xml/catalog
+</programlisting>
+    On Apple M1 based machines, use this:
+<programlisting>
+export XML_CATALOG_FILES=/opt/homebrew/etc/xml/catalog
 </programlisting>
     Without it, <command>xsltproc</command> will throw errors like this:
 <programlisting>