Doc fixes for defface and friends.

Doc fixes for defface and friends.

* lisp/cus-edit.el (custom-unlispify-remove-prefixes): Add warning.

* lisp/custom.el (defface): Doc fix.

* doc/lispref/customize.texi (Customization): Define customization more
carefully.
(Common Keywords): Add xref to Constant Variables.

* doc/lispref/display.texi (Faces): Discuss anonymous faces.
(Face Attributes): Tweak intro.
(Defining Faces): Move after the Face Attributes node.  Copyedits.
(Displaying Faces): Describe role of inheritance.

* doc/lispref/variables.texi (Defining Variables): Link to defcustom's node
instead of the higher-level Customization chapter.

Fixes: debbugs:11440