Proposed doc update for Explicit Reg Vars 3/3

[Segher Boessenkool]

On Mon, Oct 19, 2015 at 11:22:06PM -0600, Jeff Law wrote:
> WRT your hope to limit this to only uses in extended asms.  That'd be 
> nice, but that's never been an explicit limitation.  I would strongly 
> hesitate to add that limitation at this point in time.

r88265 (from 2004) made explicit that local register asm only has a
guaranteed effect on asm operands.  Since no other effect is guaranteed
we may as well not document those at all.  Bug reports do still come in
about this.


Segher