Improve apropos buffer highlighting.

Improve apropos buffer highlighting.

* lisp/apropos.el (apropos-label-face): Avoid variable-pitch face.
(apropos-accumulator): Doc fix.
(apropos-function, apropos-macro, apropos-command)
(apropos-variable, apropos-face, apropos-group, apropos-widget)
(apropos-plist): Add face property.
(apropos-symbols-internal): Fix indentation.
(apropos-print): Simplify help, and recognize apropos-multi-type.
(apropos-print-doc): Use button-type-get to extract the button's
face property.  Fill docstring (Bug#8352).