Highlight that keys in a multi-domain are sorted

As the ordering of the random numbers within a multi-field are dependent on the ordering of the keys, it is highly relevant even for the average nifty user that keys are sorted. Hence, state so in the docstrings of MultiDomain, MultiField and in the from_random method in sugar.py.

nifty6/multi_domain.py

@@ -22,9 +22,18 @@ from .utilities import frozendict, indent
 class MultiDomain(object):
     """A tuple of domains corresponding to a direct sum.
 
-    This class is the domain of the direct sum of fields defined
-    on (possibly different) domains. To make an instance
-    of this class, call `MultiDomain.make(inp)`.
+    This class is the domain of the direct sum of fields defined on (possibly
+    different) domains. To make an instance of this class, call
+    `MultiDomain.make(inp)`.
+
+    Notes
+    -----
+    For consistency and to be independent of the order of insertion, the keys
+    within a multi-domain are sorted. Hence, renaming a domain may result in it
+    being placed at a different index within a multi-domain. This is especially
+    important if a sequence of, e.g., random numbers is distributed sequentially
+    over a multi-domain. In this example, ordering keys differently will change
+    the resulting :class:`MultiField`.
     """
     _domainCache = {}