use <code> for code points, interfaces, ...

This is the PR related to Bug https://www.w3.org/Bugs/Public/show_bug.cgi?id=27197

The changes are purely editorial. I've tried to consistently used &lt;code&gt;xxx&lt;code&gt; when xxx appeared to be:
- a construct in a media format (e.g. Boxes in MP4, descriptors in TS, ...)
- a JavaScript interface (VTTCue, TextTrack ...)
I've also replaced a few "stream type" to "stream_type" in TS.

See the diff here:
http://services.w3.org/htmldiff?doc1=https%3A%2F%2Fraw.githubusercontent.com%2Fw3c%2FHTMLSourcingInbandTracks%2Fmaster%2Findex.html&doc2=https%3A%2F%2Fraw.githubusercontent.com%2Fcconcolato%2FHTMLSourcingInbandTracks%2Fbug27197%2Findex.html

If ok, please merge as soon as possible so that other fixes can now use this convention.