usb: docs: document altmode register/unregister functions

The typec_bus.rst asks for documentation of those two
functions, but they don't exist:

	./drivers/usb/typec/bus.c:1: warning: 'typec_altmode_unregister_driver' not found
	./drivers/usb/typec/bus.c:1: warning: 'typec_altmode_register_driver' not found

Also, they're not declared on bus.c but, instead, at a header
file (typec_altmode.h).

So, add documentation for both functions at the header and
change the kernel-doc markup under typec_bus.rst to point
to the right place.

While here, also place the documentation for both structs
declared on typec_altmode.h at the rst file.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>