[DOC] fix API doc and rst warnings

[szha]

Description

fix API doc problems

Checklist

Essentials

• PR's title starts with a category (e.g. [BUGFIX], [MODEL], [TUTORIAL], [FEATURE], [DOC], etc)
• Changes are complete (i.e. I finished coding on this PR)

Changes

• fix sphinx API doc warnings

[mxnet-bot]

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