[osis-core] New milestone use of elements

[Troy A. Griffitts]

> What I did was to add the mID attribute to all the elements that can be 
> listed in the milestoneSE type attribute.


I know this is silly nit-picking... but...

Is there a way to have a schema 'form' (speaking Platonic, for lack of 
my schema knowledge) that declares the attribute mID as valid.  We could 
call the form, 'milestoneable' or something-or-other, then the elements 
that are milestoneable could 'be of|inherit|get' this form.  This way we 
could have all the docs regarding the 'milestoneable' semantic at this 
point, including all the OSIS rules regarding such.  I realize they will 
just be pros, but at least they will be where someone would expect to 
find them.

	-Troy.