Merge pull request #26859 from Kumataro:fix26858

imgcodecs:gif: support IMREAD_UNCHANGED and IMREAD_GRAYSCALE #26859

Close #26858 

### Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

- [x] I agree to contribute to the project under Apache 2 License.
- [x] To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
- [x] The PR is proposed to the proper branch
- [x] There is a reference to the original bug report and related work
- [x] There is accuracy test, performance test and test data in opencv_extra repository, if applicable
      Patch to opencv_extra has the same branch name.
- [x] The feature is well documented and sample code can be built with the project CMake

modules/imgcodecs/include/opencv2/imgcodecs.hpp

@@ -413,13 +413,13 @@ can be saved using this function, with these exceptions:
 - With JPEG 2000 encoder, 8-bit unsigned (CV_8U) and 16-bit unsigned (CV_16U) images can be saved.
 - With JPEG XL encoder, 8-bit unsigned (CV_8U), 16-bit unsigned (CV_16U) and 32-bit float(CV_32F) images can be saved.
   - JPEG XL images with an alpha channel can be saved using this function.
-    To do this, create 8-bit (or 16-bit, 32-bit float) 4-channel image BGRA, where the alpha channel goes last.
-    Fully transparent pixels should have alpha set to 0, fully opaque pixels should have alpha set to 255/65535/1.0.
+    To achieve this, create an 8-bit 4-channel (CV_8UC4) / 16-bit 4-channel (CV_16UC4) / 32-bit float 4-channel (CV_32FC4) 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 255/65535/1.0.
 - With PAM encoder, 8-bit unsigned (CV_8U) and 16-bit unsigned (CV_16U) images can be saved.
 - With PNG encoder, 8-bit unsigned (CV_8U) and 16-bit unsigned (CV_16U) images can be saved.
-  - PNG images with an alpha channel can be saved using this function. To do this, create
-    8-bit (or 16-bit) 4-channel image BGRA, where the alpha channel goes last. Fully transparent pixels
-    should have alpha set to 0, fully opaque pixels should have alpha set to 255/65535 (see the code sample below).
+  - PNG 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 255/65535(see the code sample below).
 - With PGM/PPM encoder, 8-bit unsigned (CV_8U) and 16-bit unsigned (CV_16U) images can be saved.
 - With TIFF encoder, 8-bit unsigned (CV_8U), 8-bit signed (CV_8S),
                      16-bit unsigned (CV_16U), 16-bit signed (CV_16S),
@@ -428,6 +428,11 @@ can be saved using this function, with these exceptions:
   - Multiple images (vector of Mat) can be saved in TIFF format (see the code sample below).
   - 32-bit float 3-channel (CV_32FC3) TIFF images will be saved
     using the LogLuv high dynamic range encoding (4 bytes per pixel)
+- With GIF encoder, 8-bit unsigned (CV_8U) images can be saved.
+  - GIF images with an alpha channel can be saved using this function.
+    To achieve this, create an 8-bit 4-channel (CV_8UC4) 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 single-channel images (CV_8UC1) are not supported due to GIF's limitation to indexed color formats.
 
 If the image format is not supported, the image will be converted to 8-bit unsigned (CV_8U) and saved that way.