Add docstring in im2rec.py

[stu1130]

Description

• updated the docstring in im2rec.py. please refer to im2rec documentation is lacking / buggy #11884
• addressed the feedback from @aaronmarkham. please refer to Refine the documentation of im2rec #12606

Checklist

Essentials

Please feel free to remove inapplicable items for your PR.

• The PR title starts with [MXNET-$JIRA_ID], where $JIRA_ID refers to the relevant JIRA issue created (except PRs with tiny changes)
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage:
• Unit tests are added for small changes to verify correctness (e.g. adding a new operator)
• Nightly tests are added for complicated/long-running ones (e.g. changing distributed kvstore)
• Build tests will be added for build configuration changes (e.g. adding a new build option with NCCL)
• Code is well-documented:
• For user-facing API changes, API doc string has been updated.
• For new C++ functions in header files, their functionalities and arguments are documented.
• For new examples, README.md is added to explain the what the example does, the source of the dataset, expected performance on test set and reference to the original paper if applicable
• Check the API doc at http://mxnet-ci-doc.s3-accelerate.dualstack.amazonaws.com/PR-$PR_ID/$BUILD_ID/index.html
• To the my best knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change

Changes

• update the im2rec tutorial link. It now point to exact im2rec section.

Comments

N/A

[lupesko]

Thanks for adding the docstring. But why not include params and return details in the docstring?
(e.g. https://github.com/apache/incubator-mxnet/blob/master/python/mxnet/autograd.py#L52)

[sandeep-krishnamurthy]

Nice work.

@aaronmarkham - Can you please take a look. Thanks.

[aaronmarkham]

Please make the one grammar change. Otherwise this is gtg!

[aaronmarkham]

lst should have at least three parts

[stu1130]

Thanks @aaronmarkham