Display a warning message, MESSAGE.
TYPE is the warning type: either a custom group name (a symbol),
or a list of symbols whose first element is a custom group name.
(The rest of the symbols represent subcategories, for warning purposes
only, and you can use whatever symbols you like.)
LEVEL should be either :debug, :warning, :error, or :emergency
(but see `warning-minimum-level' and `warning-minimum-log-level').
Default is :warning.
:emergency -- a problem that will seriously impair Emacs operation soon
if you do not attend to it promptly.
:error -- data or circumstances that are inherently wrong.
:warning -- data or circumstances that are not inherently wrong,
but raise suspicion of a possible problem.
:debug -- info for debugging only.
BUFFER-NAME, if specified, is the name of the buffer for logging
the warning. By default, it is `*Warnings*'. If this function
has to create the buffer, it disables undo in the buffer.
See the `warnings' custom group for user customization features.
See also `warning-series', `warning-prefix-function' and
`warning-fill-prefix' for additional programming features. (fn TYPE MESSAGE &optional LEVEL BUFFER-NAME)