rpmsg: Add short description of the IOCTL defined in UAPI.

Add a description of the IOCTLs and provide information on the default
value of the source and destination addresses.

Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Reviewed-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@foss.st.com>
Link: https://lore.kernel.org/r/20210311140413.31725-4-arnaud.pouliquen@foss.st.com


Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>