DOC: Expand on usage.

Author: jseabold

pandas/core/generic.py

@@ -3159,12 +3159,14 @@ def shift(self, periods=1, freq=None, axis=0, **kwds):
  periods : int
  Number of periods to move, can be positive or negative
  freq : DateOffset, timedelta, or time rule string, optional
- Increment to use from datetools module or time rule (e.g. 'EOM')
+ Increment to use from datetools module or time rule (e.g. 'EOM').
+ See Notes.
 
  Notes
  -----
  If freq is specified then the index values are shifted but the data
- if not realigned
+ is not realigned. That is, use freq if you would like to extend the
+ index when shifting and preserve the original data.
 
  Returns
  -------