[cpp-threads] Coherence Requirement wording changes for 1.10p13

[Peter Dimov]

Hans Boehm wrote:
> I looked at this and the corresponding section of the Batty et al paper. 
> The changes look good to me.  The WriteRead issue looks like a 
> longstanding bug, with the ReadWrite issue getting more recently 
> introduced.

Looks good to me, too. I view this as a formalization of the old requirement 
that the happens-before order and the modification order should be 
"consistent" - this spells out what consistency means.