Make docblocks for Gluon BatchNorm and SyncBatchNorm consistent with the code

[rondogency]

Description

Regarding #12681 we should now write running_mean and running_var in the docblocks. The current code is using running_mean and running_var in Gluon BatchNorm and SyncBatchNorm object constructor, but the current docblock is using moving_mean and moving_var, so the docblock should be changed to be consistent with the code to avoid user confusion.

Although we should use moving_xxx for BatchNorm according the original BatchNorm paper and our backend, we should not break our old Gluon API in minor release. I have put this into MXNet 2.0 roadmap #9686 (comment), and we will make a complete fix down the line.

Checklist

Essentials

Please feel free to remove inapplicable items for your PR.

• 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

• Feature1, tests, (and when applicable, API doc)
• Feature2, tests, (and when applicable, API doc)

Comments

• If this change is a backward incompatible change, why must this change be made.
• Interesting edge cases to note here

[rondogency]

@samskalicky please review

[wkcn]

LGTM. Thank you!

[wkcn]

Merged. Thank you!