QBasicAtomicInt: fix wrong comment about non-atomic API

The public documentation for load() and store() says it's atomic,
and it is:
* using _q_value.store(newValue, std::memory_order_relaxed) in the C++11
implementation
* using a simple assignment otherwise, which is atomic (and relaxed, no
memory barriers) on all the existing C++ ABIs.

Change-Id: I40faa47120163225bd11c3a32514ac97ef8bbbd4
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>