Improve the comment for read_sequence_header.

Clarify "sequence header OBU" vs. SequenceHeader (a struct).

BUG=aomedia:1899

Change-Id: I9953187873e775d725ed18637b3b62aea54ddc57
diff --git a/av1/decoder/decodeframe.h b/av1/decoder/decodeframe.h
index dc7746c..b0109ac 100644
--- a/av1/decoder/decodeframe.h
+++ b/av1/decoder/decodeframe.h
@@ -19,9 +19,10 @@
 struct AV1Decoder;
 struct aom_read_bit_buffer;
 
-// Reads the following range of fields in the sequence header: from
-// frame_width_bits_minus_1 to enable_restoration. Reports errors by calling
-// rb->error_handler() or aom_internal_error().
+// Reads the middle part of the sequence header OBU (from
+// frame_width_bits_minus_1 to enable_restoration) into cm->seq_params (a
+// SequenceHeader). Reports errors by calling rb->error_handler() or
+// aom_internal_error().
 void read_sequence_header(AV1_COMMON *cm, struct aom_read_bit_buffer *rb);
 
 void av1_read_frame_size(struct aom_read_bit_buffer *rb, int num_bits_width,