Recent change to describe-variable

[Chong Yidong]

The completion function for describe-variable now uses the predicate

  (lambda (vv)
    (or (special-variable-p vv)
        (get vv 'variable-documentation)))

This breaks using C-h v to examine let-bound variables in Lisp
backtraces.

Also, when coding I sometimes do (setq foo 1), and use C-h v to examine
the placeholder variable later on; this now fails.

There doesn't seem to be any good reason to restrict the completion this
way.