Clarify escaping for $.

Repeat that $$ is used when a literal $ is desired.

docs/source/interactive/reference.rst

@@ -387,6 +387,8 b' 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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