Don't auto-export terms defined in informative section

[tidoust]

Is your feature request related to a problem? Please describe.
The WebRTC Stats spec reproduces the RTCStats dictionary in an informative IDL block for ease of reference. ReSpec exports the definitions contained in that IDL block as it would for a normative IDL block. This creates duplicate definitions of {{RTCStats}} (one normative and one informative), which is not ideal. Reproducing IDL blocks does not seem ideal either for sure, but as long as that remains an internal construct explained in prose and flagged as informative, that's probably not a big deal. Via w3c/webref#306 (comment) and w3c/webrtc-stats#721

Describe the solution you'd like
ReSpec and Bikeshed automatically export terms that have a certain definition type (including IDL terms). Perhaps the auto-export mechanism should only apply to normative sections? I can have a look at Webref data to see whether that change would affect a lot of definitions. That should not ;)

Note that, when given an informative IDL block, Bikeshed does not create any definition at all. That may be a bit extreme though.

An alternative could be to support the use of a data-noexport attribute on an IDL block to mean: "don't export the IDL definitions present in that block".