scripts/kernel-doc: make unknown function prototype a Warning instead of an Error

When scripts/kernel-doc cannot understand a function prototype,
it had been generating a fatal error and stopping immediately.
Make this a Warning instead of an Error and keep going.

Note that this can happen if the kernel-doc notation that is being
parsed is not actually a function prototype; maybe it's a struct or
something else, so I added "function" to the warning message to try
to make it clearer that scripts/kernel-doc is looking for a function
prototype here.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Mark Brown <broonie@kernel.org>
Acked-by: Rob Landley <rob@landley.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>