staging: pi433: add docs to packet_format and tx_start_condition enum

staging: pi433: add docs to packet_format and tx_start_condition enum

While pi433 driver deals with the nuances of the different possible
config combinations, it's hard (at first) to understand the rationale
for some of the tx/rx-related source code unless you're fairly familiar
with the rf69's inner workings.

This patch documents the expected behaviour and limits of both
packet_format and tx_start_condition enum fields.

Signed-off-by: Paulo Miguel Almeida <paulo.miguel.almeida.rodenas@gmail.com>
Link: https://lore.kernel.org/r/20211231001933.GA6779@mail.google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>