Add some docstrings to gen2.py

[archit120]

Some documentation for python bindings generator file gen2.py.

Original issue: #16339

Pull Request Readiness Checklist

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

• I agree to contribute to the project under OpenCV (BSD) License.
• To the best of my knowledge, the proposed patch is not based on a code under GPL or other license that is incompatible with OpenCV
• The PR is proposed to proper branch
• There is reference to 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

[VadimLevin]

@archit120 Thank you for the contribution! I think docstring should contain an example of usage describing expected behavior like it is done for libraries like NumPy or SciPy.
This info not only demonstrates how this function can be called but can be used with pydoctest or be converted to actual test cases.
Can you update your docs with it and add appropriate tests for python bindings generator?

[asmorkalov]

@archit120 The patch was discussed on OpenCV team weekly meeting and @alalek @asmorkalov @VadimLevin @vladimir-dudnik @vpisarev decided to reject it because it brings small additional value and introduce extra conflicts during 3.4 merge to master.