DOC: rolling_* functions: unclear docstrings

[jorisvandenbossche]

E.g. for rolling_sum: http://pandas.pydata.org/pandas-docs/dev/generated/pandas.stats.moments.rolling_sum.html#pandas.stats.moments.rolling_sum

Some issues I encountered:

• it is not mentioned the result is set to the right edge of the window.
• the center keyword is not documented.
• the explanation for freq (Frequency to conform to before computing statistic) is totally unclear to me. Conform how? It seems from the code it is doing a default resample, so this calculates a mean first?
• How are NaN's handled?

And something else, an interesting feature (and what I first thought freq would do) would be that you can specify the window size as a frequency (if dealing with a datetimeindexed series/frame) instead of a number (eg a frequency of '15min' instead of n=15 if your data are every minute).