Update commentary section.

Update commentary section.
Only require font-lock when compiling.
Define all modes conditionally.
Place all generic modes in the generic-x-modes customization group.
(generic-x-modes): New customization group.
(generic-default-modes, generic-mswindows-modes)
(generic-unix-modes, generic-other-modes): New constants.
(generic-define-mswindows-modes, generic-define-unix-modes):
Update docstrings.  Make them obsolete.
(generic-extras-enable-list): New default value.  Update
docstring.  Improve :type.  Change :set function.
(bat-generic-mode-syntax-table, rul-generic-mode-syntax-table):
Fix docstring.