Javadoc for Cache.invalidate could be more clear

[spand]

Currently the javadoc states: The behavior of this operation is undefined for an entry that is being loaded and is otherwise not present.

I had coupled the not present part with the loading part and concluded that it was in fact specified for reloads since those entries are present. #193 mentions that this is not the case and we encountered it also.

Suggestion for a clearer javadoc: The behavior of this operation is undefined for an entry that is being loaded (including both loads and reloads) or is not present. assuming this is correct.