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

[Benjamin Kosnik]

> I like the way this breaks out the write/write, read/read, ... cases!

Yeah. This was done by popular demand after discussion. Makes it much
easier to read now I think. 

Just wait till you see this with the coherence requirement list AND the
pictures!!! (Apparently the document editor needs these in dot format.)

-benjamin