Document avifRGBImage initialization requirement

This gives us an option to repurpose some of the bits of an existing
member if we ever need to add a new member.

include/avif/avif.h

diff --git a/include/avif/avif.h b/include/avif/avif.h
index 75131c5..6122353 100644
--- a/include/avif/avif.h
+++ b/include/avif/avif.h
@@ -671,6 +671,8 @@
     AVIF_CHROMA_DOWNSAMPLING_SHARP_YUV = 4     // Uses sharp yuv filter (libsharpyuv), available for 4:2:0 only, ignored for 4:2:2
 } avifChromaDownsampling;
 
+// NOTE: avifRGBImage must be initialized with avifRGBImageSetDefaults() (preferred) or memset()
+// before use.
 typedef struct avifRGBImage
 {
     uint32_t width;                        // must match associated avifImage