tools/memory-model: Weaken ctrl dependency definition in explanation.txt

The current informal control dependency definition in explanation.txt is
too broad and, as discussed, needs to be updated.

Consider the following example:

> if(READ_ONCE(x))
>   return 42;
>
> WRITE_ONCE(y, 42);
>
> return 21;

The read event determines whether the write event will be executed "at all"
- as per the current definition - but the formal LKMM does not recognize
this as a control dependency.

Introduce a new definition which includes the requirement for the second
memory access event to syntactically lie within the arm of a non-loop
conditional.

Link: https://lore.kernel.org/all/[email protected]/
Cc: Marco Elver <[email protected]>
Cc: Charalampos Mainas <[email protected]>
Cc: Pramod Bhatotia <[email protected]>
Cc: Soham Chakraborty <[email protected]>
Cc: Martin Fink <[email protected]>
Co-developed-by: Alan Stern <[email protected]>
Signed-off-by: Alan Stern <[email protected]>
Signed-off-by: Paul Heidekrüger <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>

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