Explicitly forbid reusing encoder instance (#3163)

include/avif/avif.h

diff --git a/include/avif/avif.h b/include/avif/avif.h
index 2d3d880..204827b 100644
--- a/include/avif/avif.h
+++ b/include/avif/avif.h
@@ -1415,6 +1415,9 @@
 // to reset the internal decoder back to before the first frame. Calling either
 // avifDecoderSetSource() or avifDecoderParse() will automatically Reset the decoder.
 //
+// The decoder must be destroyed once there is no need for further parsing or decoding.
+// Reusing the decoder instance for another file is not recommended. Call avifDecoderCreate() instead.
+//
 // avifDecoderSetSource() allows you not only to choose whether to parse tracks or
 // items in a file containing both, but switch between sources without having to
 // Parse again. Normally AVIF_DECODER_SOURCE_AUTO is enough for the common path.
@@ -1648,6 +1651,7 @@
 //
 // Usage / function call order is:
 // * avifEncoderCreate()
+//
 // - Still image:
 //   * avifEncoderAddImage() [exactly once]
 // - Still image grid:
@@ -1661,12 +1665,16 @@
 // - Still layered grid:
 //   * Set encoder->extraLayerCount correctly
 //   * avifEncoderAddImageGrid() ... [exactly encoder->extraLayerCount+1 times]
+//
 // * 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.
 //
+// The encoder must be destroyed after avifEncoderFinish() is called.
+// The encoder instance cannot be reused. Call avifEncoderCreate() instead.
+//
 // durationInTimescales is ignored if AVIF_ADD_IMAGE_FLAG_SINGLE is set in addImageFlags,
 // or if we are encoding a layered image.
 AVIF_API avifResult avifEncoderAddImage(avifEncoder * encoder, const avifImage * image, uint64_t durationInTimescales, avifAddImageFlags addImageFlags);