#323: invocations array constraint

This is an editorial change to emphasize the
invocations  array conceptually represents a single run of a single tool.

Change draft:

https://github.com/oasis-tcs/sarif-spec/blob/master/Documents/ChangeDrafts/Accepted/sarif-v2.0-editorial-323-invocations-array-constraints.docx

Provisional draft, with all changes merged:

https://github.com/oasis-tcs/sarif-spec/blob/master/Documents/ProvisionalDrafts/sarif-v2.0-csd02-provisional.docx

Thanks,
Larry