storage.h: document fields of header

src/storage.h

@@ -10,11 +10,17 @@ enum osmo_e1cap_capture_mode {
 
 /* header for each frame we store */
 struct osmo_e1cap_pkthdr {
+	/* Timestamp at which frame was received */
 	struct timeval ts;
+	/* length of frame data after this header */
 	uint32_t len;
+	/* line/span number on which frame was received */
 	uint8_t line_nr;
+	/* timeslot number on which frame was received */
 	uint8_t ts_nr;
+	/* see osmo_e1cap_capture_mode */
 	uint8_t capture_mode;
+	/* any optional future flags */
 	uint8_t flags;
 } __attribute__((aligned));