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 <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/staging/pi433/rf69_enum.h

···
 };
 
 enum packet_format {
+	/*
+	 * Used when the size of payload is fixed in advance. This mode of
+	 * operation may be of interest to minimize RF overhead by 1 byte as
+	 * no length byte field is required
+	 */
 	packet_length_fix,
+	/*
+	 * Used when the size of payload isn't known in advance. It requires the
+	 * transmitter to send the length byte in each packet so the receiver
+	 * would know how to operate properly
+	 */
 	packet_length_var
 };
 
 enum tx_start_condition {
+	/* the number of bytes in the FIFO exceeds FIFO_THRESHOLD */
 	fifo_level,
+	/* at least one byte in the FIFO */
 	fifo_not_empty
 };