Clarify "items"/"additionalItems" interaction.

[handrews]

In #135 the question of whether additionalItems could fail validation
if items is not defined was raised. Since items defines a default
value of {}, it is never undefined, which is clarified in this change.

[awwright]

This is an improvement, but I'm thinking the entire section could just be rewritten to something like:

If the instance is an array, then "additionalItems" is invalid if any item in the instance, that hasn't been validated with "items", is invalid

[handrews]

From an implementation perspective, I am not tracking which things have and haven't been validated. With additionalItems, I just look at the comparative lengths of items and the instance. That's the rationale for the wording that's there right now. I can therefore validate "additionalItems" without validating "items" (which is only important b/c I am implementing each keyword as a separate pure function and trying to make minimal use of other keywords in each function).

To me, your alternate wording suggests a different implementation approach that is not as useful of a design to me.

[epoberezkin]

@handrews just to remind, additionalItems can be schema, if items is an array. I guess you mean it anyway.