Update DataModule docs following property deprecations

[ananthsub]

What does this PR do?

Follow up to #8851 (comment)

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[codecov]

Codecov Report

Merging #8864 (6d8ccd4) into master (24f0124) will decrease coverage by 4%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #8864    +/-   ##
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- Coverage      93%     89%    -4%     
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  Files         169     169            
  Lines       14078   14078            
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- Hits        13052   12474   -578     
- Misses       1026    1604   +578     

[ananthsub]

@carmocca - I wasn't sure what to do about these docs that mention dm._has_setup_fit as this is also deprecated, but it's still the current behavior: https://github.com/PyTorchLightning/pytorch-lightning/blob/master/docs/source/extensions/datamodules.rst#setup

[carmocca]

I wasn't sure what to do about these docs that mention dm._has_setup_fit as this is also deprecated, but it's still the current behavior

That's fine. We will just have to remember to update that when the logic is finally changed.

[Tshimanga]

@ananthsub sorry for not getting to this sooner 🙏🏽