[XBL] Editorial: splitting section 4.3 into sub sections and cleaned up

I think parts of section 4.3 (attribute forwarding) should be split into 
sub sections. In particular, xbl:text deserves its own section, so does 
xbl:lang, and so do the type designations. In general, this whole 
section should be structured more coherently, with some better examples 
of the usage of xbl:text and xbl:lang.