Improve _complex_masking_only decorator to avoid a documentation bug

madarrays/mad_array.py

@@ -95,6 +95,7 @@ def _complex_masking_only(f):
             errmsg = 'Method not defined if masking is not complex.'
             raise ValueError(errmsg)
         return f(self)
+    decorated.__doc__ = f.__doc__
     return decorated