Unescape \= in signature docstring

[nbfalcon]

helpful--signature: unquote =, as added by help--quote-signature.

An example where this is needed is \(setf\ seq-elt\), which used to be displayed as (\=\(setf\=\ seq-elt\=\) (now (\(setf\ seq-elt\) ARG0 ARG &rest ARGS), as with describe-function).

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