Document empty environment variables not being passed to hooks under Windows.
--- a/doc/hgrc.5.txt
+++ b/doc/hgrc.5.txt
@@ -308,6 +308,11 @@ 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