doc: kernel: Add message queue & pipe to poll docs

The polling API can be used to wait on data in a FIFO, message queue,
or pipe, but the docs were not clear that message queues and pipes
are supported.

Add to the docs to make it clear message queues and pipes
can be used with the polling API.

Signed-off-by: Ben Marsh <ben.marsh@helvar.com>
1 file changed