HDRP: Decal - Add a comment

com.unity.render-pipelines.high-definition/HDRP/Material/Decal/Decal.hlsl

 	DBufferType0 MERGE_NAME(NAME, 0) = LOAD_TEXTURE2D(MERGE_NAME(TEX, 0), unCoord2);	\
 	DBufferType1 MERGE_NAME(NAME, 1) = LOAD_TEXTURE2D(MERGE_NAME(TEX, 1), unCoord2);	\
 	DBufferType2 MERGE_NAME(NAME, 2) = LOAD_TEXTURE2D(MERGE_NAME(TEX, 2), unCoord2);	\
-	DBufferType3 MERGE_NAME(NAME, 3) = LOAD_TEXTURE2D(MERGE_NAME(TEX, 3), unCoord2).xy;
+	DBufferType3 MERGE_NAME(NAME, 3) = LOAD_TEXTURE2D(MERGE_NAME(TEX, 3), unCoord2).xy; // Caution: We do .xy to match #define DBufferType3 float2 and avoid a shader warning
 
 #define ENCODE_INTO_DBUFFER(DECAL_SURFACE_DATA, NAME) EncodeIntoDBuffer(DECAL_SURFACE_DATA, MERGE_NAME(NAME,0), MERGE_NAME(NAME,1), MERGE_NAME(NAME,2), MERGE_NAME(NAME,3))
 #define DECODE_FROM_DBUFFER(NAME, DECAL_SURFACE_DATA) DecodeFromDBuffer(MERGE_NAME(NAME,0), MERGE_NAME(NAME,1), MERGE_NAME(NAME,2), MERGE_NAME(NAME,3), DECAL_SURFACE_DATA)