Add docs for datamodule hparams

[s-rog]

What does this PR do?

Fixes #9102 (which adds missing docs for #3792)

Does your PR introduce any breaking changes? If yes, please list them.

None

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Notes

Perhaps the bottom is not the best place for this?
preview

[codecov]

Codecov Report

Merging #9428 (a1db3ee) into master (c963bf6) will decrease coverage by 0%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #9428    +/-   ##
=======================================
- Coverage      93%     93%    -0%     
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  Files         179     180     +1     
  Lines       14927   15064   +137     
=======================================
+ Hits        13868   13984   +116     
- Misses       1059    1080    +21     

[akihironitta]

For easy access to the built docs: https://129007-178626720-gh.circle-artifacts.com/0/html/extensions/datamodules.html#hyperparameters-in-datamodules