--- a/mercurial/help.py
+++ b/mercurial/help.py
@@ -37,6 +37,11 @@ helptable = {
 
     'environment|env|Environment Variables':
     r'''
+HG::
+    Path to the 'hg' executable, automatically passed when running hooks
+    or external tools. Falls back to 'hg' if unset and the value can't be
+    autodetected, e.g. when Mercurial is run as a Python module.
+
 HGEDITOR::
     This is the name of the editor to use when committing. Defaults to the
     value of EDITOR.