Merge pull request #6601 from vaibhavsagar/envexpand

Clarify escaping for $.

docs/source/interactive/reference.rst

@@ -387,6 +387,8 For simple cases, you can alternatively prepend $ to a variable name::
     In [7]: !echo "A system variable: $$HOME"  # Use $$ for literal $
     A system variable: /home/fperez
 
+Note that `$$` is used to represent a literal `$`.
+
 System command aliases
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