The documentation around frequency strings is unclear
http://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.rolling.html?highlight=rolling#pandas.DataFrame.rolling and various other places in the documentation talk about something called "frequency strings", however, there doesn't seem to be any explanation of what this term means anywhere. http://pandas.pydata.org/pandas-docs/stable/timeseries.html#dateoffset-objects mentions the term again without explaining it. From the few examples I found, I now think http://pandas.pydata.org/pandas-docs/stable/timeseries.html#offset-aliases (note that this is after the dateoffset objects in the documentation) is the complete list of frequency strings, but for some reason they're called "offset aliases" here. Are those the frequency strings? If not, where are they defined?
