Updated inline documentation of EntityId::newFromPrefixedId

This was suggested by Daniel.

Currently we do have many places in the code where a numeric id is converted to an EntityId object in some way. The new (deprecated) method is supposed to be an intermediate step to get rid of all numeric ids. If it's used (we have to do this in other patches since it's used in other modules) it will make it a lot easier to find and fix code that still relies on numeric ids.