pandas.date_range documentation does not have link to acceptable 'freq' values

[nmusolino]

The API documentation for pandas.date_range does not provide a link or explanation of the acceptable values for the 'freq' parameter, e.g. 'D' for calendar day frequency.

There is a highly-upvoted Stack Overflow question entitled "pandas resample documentation" which asks a related question about pandas.Series.resample.

The documentation for pandas.date_range should include a link to the "Offset Aliases" section of the timeseries documentation.

[jreback]

see #3324

pull-requests to fix this are welcome!