Fixup API ref for atomics

[Dan Bonachea]

Current API ref for atomics include text like:

Precondition: T must be one of the approved atomic types. T must be the only type used by any in-flight atomic referencing p.
...
Performs an atomic read of the location p and returns its value in a future.

The preconditions should include that p points to an object of type T (eg p is not a null pointer, or pointing at unallocated/unaligned space).

Also, in the second sentence we should delete the word "in-flight" (and possibly copy this restriction to the overview). In particular, we don't support p being a pointer to member of a union that is sometimes accessed as uint64_t and sometimes as uint32_t. Atomic domains are permanently type-specific, not just during races.

Finally instead of "Performs ... of the location p" it should probably say "of the object at location p".

[Dan Bonachea]

New text:

T must be the only type used by any atomic referencing p for the entire lifetime
1971 of UPC++.

I think there's still a minor ambiguity here. In particular, if the user has a type like (pseudocode for a shared-memory object declaration):

union {
  uint64_t  full64;
  struct {
    uint32_t lo32;
    uint32_t hi32;
  };
} V;

If the user invokes atomics with p1==&V.full64 (and T=uint64_t), we need to clearly disallow atomics with p2==&V.hi32 (and T=uint32_t). Note in this case those are NOT the same value of p (p1 and p2 are different addresses in this case, although the referenced objects overlap).

Perhaps clarify with something like:

T must be the only type used by any atomic referencing any part of the object storage referenced by p for the entire lifetime of UPC++