DOC: Update missing data
http://pandas.pydata.org/pandas-docs/stable/10min.html?highlight=missing#missing-data makes no mention of NaT for marking missing values in datetime data.

Is that too early in the docs to mention that complication (and inf as null)? If so, we should link to http://pandas.pydata.org/pandas-docs/stable/missing_data.html#values-considered-missing