fill in missing doc string

ants/utils/pad_image.py

@@ -23,7 +23,12 @@ def pad_image(image, shape=None, pad_width=None, value=0.0, return_padvals=False
  dimension to match the largest existing dimension so that it
  has isotropic dimension
 
- pad_width : list of 
+ pad_width : list of integers or list-of-list of integers
+ How much to pad in each direction. If a single list is
+ supplied (e.g., [4,4,4]), then the image will be padded by half 
+ that amount on both sides. If a list-of-list is supplied
+ (e.g., [(0,4),(0,4),(0,4)]), then the image will be
+ padded unevenly on the different sides
 
  pad_value : scalar
  value with which image will be padded