bug#20492: 25.0.50; forward-sexp mistakes "[.,]" for symbol constituents in an Elisp string and comment

[Oleh Krehel]

See for instance this part of `hide-ifdefs' docstring (| is the point):

    to `hide|-ifdef-env'.

Pressing "C-M-f" gives:

    to `hide-ifdef-env'.|

Expected:

    to `hide-ifdef-env'|.

This behavior also works for comma, and in comments as well.

The difference isn't unimportant, since it's often useful to mark
symbol-at-point while in docstring or comment to see its value or to
copy it.