Tensorboard Docu about Hyperparams saving

[Skyy93]

What does this PR do?

Adds a bit documentation to the tensorboard logger to clarify its behavior in the case of saving hyperparameters.

Fixes #1225

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[codecov]

Codecov Report

Merging #5158 (d68af89) into master (d15f7a0) will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #5158   +/-   ##
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  Coverage      93%     93%           
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  Files         135     135           
  Lines       10006   10006           
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  Hits         9329    9329           
  Misses        677     677