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 (630db82) to head (b3d0993).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #21101      +/-   ##
==========================================
+ Coverage      39%      87%     +48%     
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  Files         266      269       +3     
  Lines       23504    23567      +63     
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+ Hits         9187    20553   +11366     
+ Misses      14317     3014   -11303