Make it possible to mark generalized variables as obsolete

Make it possible to mark generalized variables as obsolete

* doc/lispref/variables.texi (Adding Generalized Variables):
Document it.

* lisp/emacs-lisp/bytecomp.el (byte-compile-warn-obsolete): Alter
the interface so that it can also be used by generalized variable
warnings.
(byte-compile-function-warn): Adjust caller.
(byte-compile-check-variable): Adjust caller.

* lisp/emacs-lisp/gv.el (gv-get): Warn about obsolete generalized
variables (bug#49730).
(make-obsolete-generalized-variable): New function.