clarify text around statements and content-elements

[djhaynes]

Make it clear that both SACM statements and content-elements are information elements. This probably means updating the text in Section 4.2 and 4.3 as of -07 as well as making explicit structures in the Section 7.

[1] https://www.ietf.org/proceedings/interim-2016-sacm-06/minutes/minutes-interim-2016-sacm-06-201610131400-00
[2] https://www.ietf.org/mail-archive/web/sacm/current/msg04443.html
[3] https://www.ietf.org/proceedings/interim-2016-sacm-06/slides/slides-interim-2016-sacm-06-sessa-information-model-00.pdf