Update of SMP documentation

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Charm++ has some new options that make it easier to specify core layouts for SMP programs. Furthermore, the core to socket mapping varies between architectures (AMD vs Intel).
E.g., dual socket skylakes have all the even cores on socket 0 and all odd cores on socket 1.

[trquinn]

Correction to above: the skylakes on the Pleiades GPU nodes are mapped differently (0-17 on socket 0, 18-35 on socket 1) than the skylakes on Stampede2 (even cores on socket 0; odd on socket 1).