doc: add explanation to doctests [skip ci]

Author: oesteban

nipype/interfaces/ants/utils.py

@@ -104,17 +104,21 @@ class ImageMath(ANTSCommand, CopyHeaderInterface):
     ...     op2='0.005 0.999 256').cmdline
     'ImageMath 3 structural_maths.nii TruncateImageIntensity structural.nii 0.005 0.999 256'
 
+    >>> # Test the special case of PadImage. In this case, copy_header is disabled
+    >>> # because the header cannot be copied from inputs to ouptuts.
+    >>> # When operation is PadImage, the copy_header is set to False automatically:
     >>> pad = ImageMath(
     ...     op1='structural.nii',
-    ...     operation='PadImage',
-    ...     op2='0.005 0.999 256')
+    ...     operation='PadImage')
     >>> pad.inputs.copy_header
     False
 
+    >>> # While operation remains as PadImage, copy_header will not accept True
     >>> pad.inputs.copy_header = True
     >>> pad.inputs.copy_header
     False
 
+    >>> # For any other operation, copy_header gets its default behavior back.
     >>> pad.inputs.operation = "ME"
     >>> pad.inputs.copy_header = True
     >>> pad.inputs.copy_header