Move generalized variable documentation from misc/cl.texi to lispref

Move generalized variable documentation from misc/cl.texi to lispref

* doc/lispref/variables.texi (Generalized Variables): New section,
adapted from misc/cl.texi.

* doc/lispref/elisp.texi (Top): Add Generalized Variables to menu.

* doc/lispref/lists.texi (List Elements, List Variables):
Mention generalized variables.

* doc/misc/cl.texi (Control Structure): Update for setf now being in core.
(Setf Extensions): Rename from Basic Setf.  Move much of the
former content to lispref/variables.texi.
(Modify Macros): Move pop, push details to lispref/variables.texi.
(Customizing Setf): Copyedits for setf etc being in core.
(Modify Macros, Efficiency Concerns, Porting Common Lisp):
Further namespaces updates.