extended asm docs - re-written

[dw]

> It's a lot to review. 
It certainly is, and I appreciate you taking the time.

> That'd be input values, not variables.
True.  Fixed.

> The rest is so vage and uses
> so many weasel words as to be useless.
It is a common documentation standard to begin with brief descriptions 
of the various pieces, and then to provide more detail for each piece.  
This allows people to get an overview of the concept, and then delve 
into the details.  Section 6.41.1 goes on in some detail on the 
specifics of volatile.

However, I'm absolutely open to some replacement text.

> If it's a "control flow only block", how come it has clobbers?
The fact that asm goto blocks can have clobbers is laid out in the 
existing docs.  However, I have removed the word "only" as this could 
indeed be confusing.

> Will it?

Umm.  Oops?

I have fixed all of these points and re-generated the docs.

Thanks and keep them coming!

dw