doc(imgcodecs): update 16-bit support and opaque alpha channel behavior for AVIF(4.x)

- Reading and writing 16-bit AVIF images requires libavif v1.4.0+
- Note that if the alpha plane contains only fully opaque pixels, the destination saves only color channels due to a libavif limitation.

Port from https://github.com/opencv/opencv/pull/30519

modules/imgcodecs/include/opencv2/imgcodecs.hpp

@@ -552,10 +552,11 @@ can be saved using this function, with these exceptions:
     Fully transparent pixels should have an alpha value of 0, while fully opaque pixels should have an alpha value of 255.
   - 8-bit single-channel images (CV_8UC1) are not supported due to GIF's limitation to indexed color formats.
 - With AVIF encoder, 8-bit unsigned (CV_8U) and 16-bit unsigned (CV_16U) images can be saved.
-  - CV_16U images can be saved as only 10-bit or 12-bit (not 16-bit). See IMWRITE_AVIF_DEPTH.
+  - CV_16U images can be saved as only 10-bit or 12-bit (and 16-bit with libavif 1.4.0+). See @ref IMWRITE_AVIF_DEPTH.
   - AVIF images with an alpha channel can be saved using this function.
     To achieve this, create an 8-bit 4-channel (CV_8UC4) / 16-bit 4-channel (CV_16UC4) BGRA image, ensuring the alpha channel is the last component.
-    Fully transparent pixels should have an alpha value of 0, while fully opaque pixels should have an alpha value of 255 (8-bit) / 1023 (10-bit) / 4095 (12-bit) (see the code sample below).
+    Fully transparent pixels should have an alpha value of 0, while fully opaque pixels should have an alpha value of 255 (8-bit) / 1023 (10-bit) / 4095 (12-bit) / 65535 (16-bit) (see the code sample below).
+    Note that if the alpha plane contains only fully opaque pixels, the destination saves only color channels due to a libavif limitation.
 
 If the image format is not supported, the image will be converted to 8-bit unsigned (CV_8U) and saved that way.