Treegrid Design Pattern: Editorial Corrections per Jul 16 Feedback in…

… issue 91

Per feedback from #annabbott in issue #91, made multiple editorial revisions.
Made additional changes to phrasing to be more consistent with other patterns.
Removed first two sentences of first note in roles, states, and properties.
They stated that row and gridcell roles were not needed on treegrids built from HTML tables; that is currently not consistent with the ARIA in HTML spec.