Notes contain normative requirements

[chrisn]

Following discussion in #521, we should review all the notes in the spec, as many currently contain RFC 2119 words. Should these still be notes if they place normative requirements on implementations? Some notes place requirements on content authors (see #531).

[joeyparrish]

Solving #521 would take care of one specific, complicated normative requirement in a note. A full audit would be required to close this issue afterward.