Create new Edebug spec for docstrings and use it in closures

Create new Edebug spec for docstrings and use it in closures

Since (:documentation FORM) can be used to create a docstring
when lexical-binding is on, allow for that possibility in Edebug
specs (bug#24773).
* lisp/emacs-lisp/edebug.el: Define an Edebug spec for docstrings
called lambda-doc and modify the Edebug specs for defun and
defmacro to use it.
(edebug-instrument-function): Check for generic functions first,
to fix bug where edebug-step-in didn't work on methods now that
cl-defgeneric has an Edebug spec.
* lisp/subr.el (lambda): Modify Edebug spec to use lambda-doc.
* lisp/emacs-lisp/cl-generic.el (cl-defgeneric): Add Edebug spec
(bug#27747).
(cl-defmethod): Use lambda-doc in Edebug spec.
* lisp/emacs-lisp/cl-macs.el: Modify Edebug spec for
cl-declarations-or-string to use lambda-doc, and modify Edebug
spec for cl-lambda-expr to use cl-declarations-or-string.
* lisp/emacs-lisp/pcase.el (pcase-lambda): Modify Edebug spec to
use lambda-doc, as well as &define and def-body which are
necessary for using Edebug on code wrapped by lambda.
* lisp/emacs-lisp/generator.el (iter-defun, iter-lambda): Add
Edebug specs.