Clarify the error return for aom_rb_read_uvlc()

aom_rb_read_uvlc() returns UINT32_MAX for only one kind of error: the
input is too long (has 32 or more leading zero bits).

Change-Id: I06bb9143d086b839619da9156f584ff6e1d56227

aom_dsp/bitreader_buffer.h

diff --git a/aom_dsp/bitreader_buffer.h b/aom_dsp/bitreader_buffer.h
index 15d7119..0112fb4 100644
--- a/aom_dsp/bitreader_buffer.h
+++ b/aom_dsp/bitreader_buffer.h
@@ -39,7 +39,8 @@
 int aom_rb_read_literal(struct aom_read_bit_buffer *rb, int bits);
 
 // Reads a variable length unsigned integer. Valid range is 0..UINT32_MAX - 1.
-// Returns UINT32_MAX on error.
+// Returns UINT32_MAX if the input is too long (has 32 or more leading zero
+// bits).
 uint32_t aom_rb_read_uvlc(struct aom_read_bit_buffer *rb);
 
 #if CONFIG_AV1_DECODER