Probably no documentation necessary. This was a regression in valgrind 3.8.0 which was never released before in RHEL6. The previous version in RHEL6 was based on 3.6.0.
x86_64 programs using some variants of the xaddb instruction would not run under valgrind. This was a regression introduced in valgrind 3.8.0. Such programs would run normally under valgrind before. With this fix they again run under valgrind without errors.