Changed comment syntax

At a meeting of the ERB, with all present except Magliery and Hollander,
it was resolved unanimously to change XML comment syntax so that 
comments begin '<--*' and end '*-->'.  We will still require, pending
clarification of this by WG8, that for compatibility, neither '--' nor
'--*' can appear in the body of comment; we are hopeful that the '--'
restriction can be lifted soon.  This will require changes to section 2.5,
and to productions 19 and 21. - Tim