[Ada] User Guide?

[dewar@gnat.com]

<<I think this should be achievable with the approach we have in mind for
the users guide. We will take care of the minor issues for gnat_rm.texi.
>>

Just so no one is confused, we are coordinated with Florian here, and 
"the approach we have in mind" is exactly the approach that Florian has
been persuing (at least that's my understanding, and we will make sure
that this is all sorted out right away).