sup and sub in sup and sub

[mnot]

Revision 3.0.0 of the RNC added sub and sup into the content models of sub and sup. This needs to be documented.

[stpeter]

FWIW, Henrik has tested up to three levels of sup within sup or sup within sub and it rendered fine. The intent is that this will not provide anything close to complete math support, only the right level of support for subscripts and superscripts so that we can easily document crypto algorithms.

[jrlevine]

added to 7991bis