Update fp16 docs: Block.cast is inplace

[Kh4L]

Signed-off-by: Serge Panev spanev@nvidia.com

Description

Update fp16 docs: Block.cast is inplace

Checklist

Essentials

• Changes are complete (i.e. I finished coding on this PR)
• 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

• Doc, 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

[wkcn]

Thank you for the fix! LGTM