Add comment in avif.h for avifRWData functions (#925)

include/avif/avif.h

diff --git a/include/avif/avif.h b/include/avif/avif.h
index 64c1b05..b887eab 100644
--- a/include/avif/avif.h
+++ b/include/avif/avif.h
@@ -178,6 +178,7 @@
 #define AVIF_DATA_EMPTY { NULL, 0 }
 // clang-format on
 
+// The avifRWData input must be zero-initialized before being manipulated with these functions.
 AVIF_API void avifRWDataRealloc(avifRWData * raw, size_t newSize);
 AVIF_API void avifRWDataSet(avifRWData * raw, const uint8_t * data, size_t len);
 AVIF_API void avifRWDataFree(avifRWData * raw);