[doc] Update booster user documents.

[Fridge003]

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#4628

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• Rewrite some docstrings in booster.py
• Update user documents of booster, including booster_api.md, booster_checkpoint.md and booster_plugins.md

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[github-actions]

The code coverage for the changed files is 85%.

Click me to view the complete report

Name                                Stmts   Miss  Cover
-------------------------------------------------------
colossalai/booster/booster.py          69     12    83%
colossalai/checkpoint_io/utils.py     321     46    86%
-------------------------------------------------------
TOTAL                                 390     58    85%

[github-actions]

The code coverage for the changed files is 81%.

Click me to view the complete report

Name                                Stmts   Miss  Cover
-------------------------------------------------------
colossalai/booster/booster.py          69     12    83%
colossalai/checkpoint_io/utils.py     321     61    81%
-------------------------------------------------------
TOTAL                                 390     73    81%

[github-actions]

The code coverage for the changed files is 83%.

Click me to view the complete report

Name                            Stmts   Miss  Cover
---------------------------------------------------
colossalai/booster/booster.py      69     12    83%
---------------------------------------------------
TOTAL                              69     12    83%