Improve comment for aom_decode_frame_from_obus()

Document that aom_decode_frame_from_obus() sets *p_data_end on success
only.

av1/decoder/obu.c

diff --git a/av1/decoder/obu.c b/av1/decoder/obu.c
index 973619e..0be90a4 100644
--- a/av1/decoder/obu.c
+++ b/av1/decoder/obu.c
@@ -1005,9 +1005,9 @@
   return sz;
 }
 
-// On success, returns a boolean that indicates whether the decoding of the
-// current frame is finished. On failure, sets cm->error.error_code and
-// returns -1.
+// On success, sets *p_data_end and returns a boolean that indicates whether
+// the decoding of the current frame is finished. On failure, sets
+// cm->error.error_code and returns -1.
 int aom_decode_frame_from_obus(struct AV1Decoder *pbi, const uint8_t *data,
                                const uint8_t *data_end,
                                const uint8_t **p_data_end) {