:package-version keyword.
(custom-add-package-version): New function. Sets value of new
property 'custom-package-version from :package-version keyword.
+ (defcustom): Create Common Keywords section in docstring.
+ (defface, defgroup): Replace definitions of a select few keywords
+ with a reference to the Common Keywords in defcustom.
+ (defcustom, defface, defgroup): Replace reference to Customization
+ chapter in manual with hyperlink.
* cus-edit.el (customize-package-emacs-version-alist): New
variable.