Document: Memoize entity list allocation

[XANOZOID]

Commit in question

#574 -> Updated Scene to return empty containers

Comment

I feel like it makes sense to use some form of memoization
If we're querying the existing entity types and someone attempts to get a qualifying Entity, then cache the result as empty
much like adding an entity of that type, but with no actual allocation other than the array

Ben Morris's suggestion:

Users shouldn't be modifying these return values, that would already cause problems. We could enforce that with a read-only abstract but I don't think it's necessary.