(help-mode): Use define-derived-mode.

(help-mode): Use define-derived-mode.
(describe-mode): Add optional `buffer' arg.
Use it instead of going through help-xref-mode.
Avoid doubling the word `minor' when prettifying.
(describe-function-1): List the corresponding key bindings.
(describe-variable): Say if the var is automatically buffer-local.
If the source is `loaddefs.el', look for the real source.
(help-xref-mode): Remove.