Make checkdoc check cl-lib function docstrings

Make checkdoc check cl-lib function docstrings

* lisp/emacs-lisp/checkdoc.el (checkdoc-this-string-valid-engine):
Remove calls to delete-region to avoid deleting final " (bug#26328).
* lisp/emacs-lisp/checkdoc.el (checkdoc--next-docstring)
(checkdoc-defun-info): Include cl-defun, cl-defgeneric,
cl-defmethod.
(checkdoc-this-string-valid-engine): Add cl-lib supported
keywords.
(checkdoc-defun-info): Ensure function parameters are a
"flat" list (bug#37034).