More customization-related documentation updates.

More customization-related documentation updates.

* doc/emacs/custom.texi (Specifying File Variables): The mode: keyword
doesn't have to be first anymore.  Add example of specifying minor
modes.
(Directory Variables): Simplify example.  Mention application to
non-file buffers.
(Disabling): Use "initialization file" terminology.
(Init Examples): Fix hook example.

* doc/lispref/variables.texi (Directory Local Variables): Document
hack-dir-local-variables-non-file-buffer.

* lisp/files.el (hack-dir-local-variables-non-file-buffer): Add doc.
(hack-one-local-variable-eval-safep): Allow 0 arg for minor mode
functions, along with 1 and -1.