Add comment

src/tiff/tiffread.c

@@ -341,6 +341,7 @@ internal TiffSampleBits read_bits_per_sample(const TiffReader *reader,
   TiffSampleBits bits = {0};
   memcpy(&bits, bits_per_sample, TIFF_SHORT_BYTE_COUNT * main_samples);
 
+  // Set missing samples if image doesn't have all RGB samples
   if (main_samples < RGB_SAMPLE_COUNT) {
     u64 count = RGB_SAMPLE_COUNT - main_samples;
     for (u64 i = 0; i < count; ++i) {