Clarify that statement ID Is always required when putting a statement

Addresses https://github.com/adlnet/xAPI-Spec/issues/364 and works towards https://github.com/adlnet/xAPI-Spec/issues/209

Note that I've set the value as "Required" rather than "yes", as I anticipate we'll be changing all the occurrences of "yes" in these columns to "Required" as part of https://github.com/adlnet/xAPI-Spec/issues/209 anyway.