tools/memory-model/Documentation: Add plain accesses and data races to explanation.txt

This patch updates the Linux Kernel Memory Model's explanation.txt
file by adding a section devoted to the model's handling of plain
accesses and data-race detection.

Signed-off-by: Alan Stern <[email protected]>
Acked-by: Andrea Parri <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>

Author: AlanStern