help/config: note 64-bit Windows registry key used with 32-bit Python

Before this patch the only place that documented the use of a different
registry key was on the wiki page for installing on Windows from
sources, while most users will use a pre-packaged installer and the
supplied help files.

This patch documents in the supplied help files that an alternate
registry key is used for the installation/system configuration file when
using 32-bit Python on a 64-bit Windows.

mercurial/help/config.txt

@@ -85,6 +85,9 b' ones.'
     be read.  Mercurial checks each of these locations in the specified
     order until one or more configuration files are detected.
 
+.. note:: The registry key ``HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Mercurial``
+   is used when running 32-bit Python on 64-bit Windows.
+
 Syntax
 ======