Update docs for ModelCheckpoint save_last

[awaelchli]

What does this PR do?

The docs for the ModelCheckpoint argument save_last were misleading.
This PR updates the description of how the feature works.
Addresses partially #11805 (comment)

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Did you have fun?

I made sure I had fun coding 🙃

cc @Borda @rohitgr7 @carmocca @awaelchli @ninginthecloud @jjenniferdai

[carmocca]

Thanks!