DOC deprecation warning to_datetime

[glemaitre]

I was wondering if in the documentation on Timeseries, the occurrences to_datetime should be changed to to_pydatetime.

If this is the case I can make a PR.

[gfyoung]

@glemaitre : All references to to_datetime should have been removed due to #18648. Any remaining ones should be to pd.to_datetime. If not, a PR is welcome!

[jreback]

closing as this is done @gfyoung pls check in any event.