Remove non-option variable handling from customize-apropos

Remove non-option variable handling from customize-apropos
and enable use of word lists for customize-apropos-options etc.

* lisp/apropos.el (apropos-read-pattern): Make prompt less cryptic.

* lisp/cus-edit.el (customize-apropos, customize-apropos-options):
Disable matching of non-option variables.
(customize-option, customize-option-other-window)
(customize-changed-options): Doc fix.
(customize-apropos-options, customize-apropos-faces)
(customize-apropos-groups): Use apropos-read-pattern.