doc: rmpsg: Update with rpmsg_endpoint

It seems the documentation was not updated when `rpmsg_sendto`
and related switched from `rpmsg_channel` to `rpmsg_endpoint`.
This change updates the proper calls, text, and the sample.

Signed-off-by: Adrien Leravat <adrien.leravat@gmail.com>
Link: https://lore.kernel.org/r/20231203200606.255302-1-adrien.leravat@gmail.com
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>