Fix documentation for DisparityFilter disparity map types

[krishnavasnani07]

This PR fixes incorrect documentation in ximgproc::DisparityFilter regarding
the expected type of disparity_map_right and clarifies the output disparity
map format. No functional or behavioral changes are included.

This is a documentation-only change.

Pull Request Readiness Checklist

• I agree to contribute to the project under Apache 2 License.
• 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
• The PR is proposed to the proper branch
• There is a reference to the original bug report and related work
• There is accuracy test, performance test and test data in opencv_extra repository, if applicable
  Patch to opencv_extra has the same branch name.
• The feature is well documented and sample code can be built with the project CMake