DOC: document that DataFrame and Series round() only affect numeric columns

[sfc-gh-mvashishtha]

DataFrame.round and Series.round seem to ignore non-numeric columns, but their documentation doesn't mention that.

[sfc-gh-mvashishtha]

edit: round() seems to ignore all non-numeric columns, which is fine. However, we should document that. I was a little surprised by this behavior for Timedelta columns because Timedelta.round does something different than Series.round.

[rhshadrach]

Thanks for the report. Makes sense to me - PRs to improve the docs here are welcome!