Update docs on arg train_dataloader in fit

[SkafteNicki]

What does this PR do?

Fixes #1959 (comment)
Updates docstring of fit to mention that train_dataloader now can take in multiple dataloaders.

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[codecov]

Codecov Report

Merging #6076 (f4ce862) into master (0b27147) will decrease coverage by 2%.
The diff coverage is 89%.

@@           Coverage Diff           @@
##           master   #6076    +/-   ##
=======================================
- Coverage      93%     91%    -2%     
=======================================
  Files         160     159     -1     
  Lines       11402   11365    -37     
=======================================
- Hits        10654   10341   -313     
- Misses        748    1024   +276     

[rohitgr7]

maybe add some examples and update type hints here too: https://github.com/PyTorchLightning/pytorch-lightning/blob/0b271474e5cc35a40009559d6c4488ab3b02723e/pytorch_lightning/core/hooks.py#L386??