DOC: to_datetime works real differently on DataFrames vs other datatypes
#### Location of the documentation

* https://pandas.pydata.org/docs/reference/api/pandas.to_datetime.html
* https://pandas.pydata.org/docs/dev/reference/api/pandas.to_datetime.html?highlight=to_datetime

#### Documentation problem

The `to_datetime` method does something wildly different on strings or Series vs DataFrames. This caught me by surprise and I think it would help if the documentation reflected this more clearly.

#### Suggested fix for documentation

Something along the lines of:

For most datatypes given to the `to_datetime` method it creates datetime objects out of strings. If `to_datetime` is given a dataframe it does something quite different: it combines columns that contain year, month and day information _into_ datetime objects.
