(thing-at-point 'list) return nil if no list at point

(thing-at-point 'list) return nil if no list at point

* lisp/thingatpt.el (thing-at-point-bounds-of-list-at-point):
Check first if we are at the beginning of a top-level sexp (Bug#24627).
If point is inside a comment or string, look for a list out of the
comment/string.
Escape '[' in doc string.
* test/lisp/thingatpt-tests.el (thing-at-point-bug24627): Update
expected test result as pass.