Add a note in the spec about causality cycles/out-of-thin air reads

This memory model appears to have inherited from C++ the issue of allowing causality cycles (a.k.a. out-of-thin-air reads). For example, the following program (in pseudo-code):

Thread 1:
  r = load_relaxed(x);
  store_relaxed(y, r);
Thread 2:
  r2 = load_relaxed(y);
  store_relaxed(x, r2);

can end with any value in both x and y. This is both unwanted (I don't know of any hardware that can actually write 42 in both x and y, with no reference to 42 anywhere in the program) and rather fundamental to the C++ style of axiomatic models. Fixing this in the model is way out-of-scope (it is a hard problem that academia has been trying to fix for several years, and the only solution I consider satisfactory rewrites the entire model in a different style), but we could include a comment similar to that in the C++ spec (note 9 in section 29.3 of http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4296.pdf):

9 Implementations should ensure that no “out-of-thin-air” values are computed that circularly depend on their own computation.
[ Note: For example, with x and y initially zero,
// Thread 1:
r1 = y.load(memory_order_relaxed);
x.store(r1, memory_order_relaxed);
// Thread 2:
r2 = x.load(memory_order_relaxed);
y.store(r2, memory_order_relaxed);
should not produce r1 == r2 == 42, since the store of 42 to y is only possible if the store to x stores 42, which circularly depends on the store to y storing 42. Note that without this restriction, such an execution ispossible. —endnote]

Beginne mit Issue #14 und vergleiche die Behandlung von relaxed Loads und Stores durch das Modell mit C++ N4296 §29.3 note 9. Als erledigt gilt die Aufgabe, wenn die Spezifikation einen klaren Hinweis auf Kausalitätszyklen und out-of-thin-air values enthält und dabei das mitgelieferte Zwei-Thread-Beispiel oder ein gleichwertiges Beispiel verwendet.