--- a/doc/hgrc.5.txt
+++ b/doc/hgrc.5.txt
@@ -327,6 +327,11 @@ hooks::
   Also, hooks like "commit" will be called in all contexts that
   generate a commit (eg. tag) and not just the commit command.
 
+  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