Update args, kwargs doc for load_from_checkpoint()

[yukw777]

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• Was this discussed/approved via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure to update the docs?

What does this PR do?

Fixes docs for load_from_checkpoint() as discussed in #1807.

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[codecov]

Codecov Report

Merging #1839 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #1839   +/-   ##
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  Coverage      88%     88%           
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  Files          71      71           
  Lines        4474    4474           
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  Hits         3929    3929           
  Misses        545     545