more compact lang ref

Surely some efficiencies are possible but I'm puzzled by the specific
reference to univ-atts and global-atts because they seem to be pretty
efficiently presented:

univ-atts attribute group (includes select-atts, id-atts, and
localization-atts groups) 
A set of related attributes, described in _univ-atts attribute group_


global-atts attribute group (xtrf, xtrc) 
A set of related attributes, described in _global-atts attribute group_


(I used underscore chars to delimit links here.)