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.

Reviewed docs on operators, library, sugar. Fixed bug in makeOp due to the recent BlochdiagonalOperator interface change

The reason for this is the following. MultiFields behave very similar to
dictionaries except for the fact that dictionaries are mutable and MultiFields
are immutable. Therefore, MultiField.update() needs to return a new object. That
is different to the dictionary.update() functionality and therefore unintuitive.

MultiField.union() can do the exact same thing as update() used to do.