Document avifEncoderAddImage() a bit more

Document the lifetime of the image passed to avifEncoderAddImage().

Suggested by Maryla Ustarroz-Calonge.

include/avif/avif.h

diff --git a/include/avif/avif.h b/include/avif/avif.h
index 871eaae..75131c5 100644
--- a/include/avif/avif.h
+++ b/include/avif/avif.h
@@ -1225,6 +1225,8 @@
 // * avifEncoderFinish()
 // * avifEncoderDestroy()
 //
+// The image passed to avifEncoderAddImage() or avifEncoderAddImageGrid() is encoded during the
+// call (which may be slow) and can be freed after the function returns.
 
 // durationInTimescales is ignored if AVIF_ADD_IMAGE_FLAG_SINGLE is set in addImageFlags,
 // or if we are encoding a layered image.