Document tensorboard experiment property

[SkafteNicki]

What does this PR do?

Fixes #19784
The experiment property is currently missing to be documented:
https://lightning.ai/docs/pytorch/stable/api/lightning.pytorch.loggers.tensorboard.html#module-lightning.pytorch.loggers.tensorboard
adding it and improving docs.

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📚 Documentation preview 📚: https://pytorch-lightning--21101.org.readthedocs.build/en/21101/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87%. Comparing base (1fc077b) to head (46f9217).
⚠️ Report is 26 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #21101   +/-   ##
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  Coverage      87%      87%           
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  Files         269      269           
  Lines       23656    23663    +7     
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+ Hits        20633    20640    +7     
  Misses       3023     3023