Update HTML5 processing description
Update HTML5 processing description

The initial version of the topic from 656ad1f was based on the XHTML equivalent, but since release 2.2 made HTML5 a standalone plug-in, and release 2.4 removed the remaining dependencies between HTML5 and XHTML, references to a "common routine" and XHMLT target were no longer correct.

Signed-off-by: Roger Sheen <a61a1add71dadc74037ae5e44ee30713db9a7451@infotexture.net>

infotexture/docs,shaneataylor/docs,dita-ot/docs,shaneataylor/docs,infotexture/docs