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.

Similar to dict.update(other_dict)