refresh LightningModule API docs

[awaelchli]

What does this PR do?

This PR includes several improvements to the API docs in lightning.py.

• Added missing docs to public methods and properties (the lack thereof was brought to our attention by @ananthsub)
• Fixed typos
• Fixed whitespace leading to malformatted html
• Fixed linking between methods and attributes to improve navigation (where possible and appropriate)

The build of the html can be viewed here or by going to the workflows ci-tests -> click on tab "artifacts" -> navigate to html/api/pytorch_lightning.core.lightning.html

Before submitting

• Was this discussed/approved via a GitHub issue? (not for typos and docs)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you make sure to update the documentation with your changes? (if necessary)
• Did you write any new necessary tests? (not for typos and docs)
• Did you verify new and existing tests pass locally with your changes?
• Did you update the CHANGELOG? (not for typos, docs, test updates, or internal minor changes/refactorings)

PR review

Anyone in the community is free to review the PR once the tests have passed.
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Did you have fun?

I made sure I had fun coding 🙃

[codecov]

Codecov Report

Merging #8276 (4265496) into master (8193bae) will decrease coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #8276   +/-   ##
======================================
- Coverage      93%     92%   -0%     
======================================
  Files         212     212           
  Lines       13720   13720           
======================================
- Hits        12751   12684   -67     
- Misses        969    1036   +67     

[SeanNaren]

Thanks @awaelchli :)

[kaushikb11]

Great!