Add doc-strings to some cl float parameters.

Add doc-strings to some cl float parameters.

* lisp/emacs-lisp/cl-extra.el (cl-float-limits): Add doc string.
* lisp/emacs-lisp/cl.el (most-positive-float, most-negative-float)
(least-positive-float, least-negative-float)
(least-positive-normalized-float, least-negative-normalized-float)
(float-epsilon, float-negative-epsilon): Add doc-strings,
based on those in cl.texi.

* lisp/emacs-lisp/cl-extra.el (most-positive-float, most-negative-float)
(least-positive-float, least-negative-float)
(least-positive-normalized-float, least-negative-normalized-float)
(float-epsilon, float-negative-epsilon):
Remove unnecessary declarations.