Add documentation note for TQDMProgressBar

[NishantDahal]

What does this PR do?

This PR adds a note to the documentation of TQDMProgressBar to clarify the behavior of the smoothing option. The note explains that the smoothing option has no effect when using the default implementation of TQDMProgressBar and suggests a possible solution.

Fixes #20003

• Was this discussed/agreed via a GitHub issue? (not for typos and docs) -> Yes, discussed in issue Smoothing in tqdm progress bar has no effect #20003
• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you make sure to update the documentation with your changes? (if necessary). -> Yes
• Did you write any new necessary tests? (not for typos and docs). -> No
• Did you verify new and existing tests pass locally with your changes?
• Did you list all the breaking changes introduced by this pull request? -> No
• Did you update the CHANGELOG? (not for typos, docs, test updates, or minor internal changes/refactors). -> Yes

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📚 Documentation preview 📚: https://pytorch-lightning--20198.org.readthedocs.build/en/20198/