Update LearningRateMonitor docs and tests for log_weight_decay

[Peiffap]

What does this PR do?

Fixes #19798.

Adds a description for the log_weight_decay argument for LearningRateMonitor, fixes another miscellaneous typo in the docs for that class, and adds an additional assert statement to the tests for LearningRateMonitor to ensure no weight decay values are tracked if log_weight_decay=False (by analogy with the tests for (log_momentum=False).

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📚 Documentation preview 📚: https://pytorch-lightning--19805.org.readthedocs.build/en/19805/

[Peiffap]

@Borda Apologies for having to ping you on this; could you share the reason this PR is being held back?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 53%. Comparing base (d76feef) to head (c812b20).

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #19805      +/-   ##
==========================================
- Coverage      84%      53%     -31%     
==========================================
  Files         426      418       -8     
  Lines       35233    35080     -153     
==========================================
- Hits        29501    18647   -10854     
- Misses       5732    16433   +10701