Add note on accessing last and best checkpoint

[SkafteNicki]

What does this PR do?

Fixes #14740

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📚 Documentation preview 📚: https://pytorch-lightning--21241.org.readthedocs.build/en/21241/

[deependujha]

The CI failure looks like a timeout. The logs don’t explicitly say so, but the job ran for 57 minutes, and the timeout was recently changed from 75 to 55 minutes. check pr