extended asm docs - re-written

[dw]

> Maybe it could go on the GCC wiki, where it can be corrected as
> necessary to get it into shape.

While the process of moving the text back and forth between texi and 
wiki might be a bit tedious, I'm sure this could be done.  And honestly, 
I'd be worried that this might get dropped into the wiki and forgotten.

Still, it seems like the people who are doing the reviewing should make 
this call.  If they prefer editing in place, let's do it.

dw