Document allowed NULL input in functions? #645

In many functions we allow for a NULL input since this is a legit output in case of failure of some colony actions (I don't recall just now which) so that we avoid breaking scripts or more importantly wrong inferences when some colonies have NULL status. We have sometimes documented this, but mostly not. @Jana pointed if we need to document this at janaobsteter#2 (comment). This issue is to flag this and deal with it at a later point.