Merge branch 'fix_endomorphic_doc' into 'NIFTy_5'

improve docstring

See merge request ift/nifty-dev!168

nifty5/operators/endomorphic_operator.py

@@ -23,9 +23,8 @@ from .linear_operator import LinearOperator
 class EndomorphicOperator(LinearOperator):
     """ NIFTy class for endomorphic operators.
 
-    The  NIFTy EndomorphicOperator class is a class derived from the
-    LinearOperator. By definition, domain and target are the same in
-    EndomorphicOperator.
+    The  NIFTy EndomorphicOperator describes linear operators with identical
+    domain and target.
     """
 
     @property