Update LearningRateMonitor docs and tests to mention 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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