modified domain.local_shape's docstring

It is rendered in the docs before domain.shape and as such hard
to understand, because it explains two concepts (shape, MPI data
splitting) at once.

Therefore I added the hint that it is mainly relevant for MPI
and that the docstring of domain.shape explains the shape concept
in general.

nifty5/domains/domain.py

@@ -86,7 +86,10 @@ class Domain(metaclass=NiftyMeta):
 
     @property
     def local_shape(self):
-        """tuple of int: number of pixels along each axis on the local task
+        """tuple of int: number of pixels along each axis on the local task,
+        mainly relevant for MPI.
+
+        See :meth:`.shape()` for general explanation of property.
 
         The shape of the array-like object required to store information
         defined on part of the domain which is stored on the local MPI task.