Clarify signaling section

[valer1an]

Signaling added in #26 could be confusing. DASH §5.10.3.3.1 mentions that "Event message boxes with scheme identifier and value pairs that are not defined in the MPD should not be present. If a DASH Client detects an event message box with a scheme that is not defined in MPD, the client is expected to ignore it." Some implementation might therefore prefer using "https://aomedia.org/emsg/ID3" rather than "urn:aomedia:cmaf:id3" for schemeIdUri to signal ID3 in a DASH manifest.