Clarify what clients should do if they see an unknown/invalid expression for a known annotation term.

In ODATA-786 we added new path expressions for OData 4.01, which brings up a question of what clients should do if they get an expression that they can't understand. We already say clients should ignore terms they don't understand, but we need to provide direction on what clients should do if they don't understand the expression. Or we need to make, i.e., the new expressions added to ODATA-786 only supported for clients that request ODATA 4.01 or greater.