Document empty environment variables not being passed to hooks under Windows.

doc/hgrc.5.txt

@@ -308,6 +308,11 b' hooks::'
   did not have a "HG_" prefix.  The old unprefixed names are no longer
   provided in the environment.
 
+  Note2: Environment variables with empty values may not be passed to
+  hooks on platforms like Windows. For instance, $HG_PARENT2 will
+  not be available under Windows for non-merge changesets while being
+  set to an empty value under Unix-like systems.
+
   The syntax for Python hooks is as follows:
 
     hookname = python:modulename.submodule.callable