[PATCH 2/2] manual: Document __libc_single_threaded

[Florian Weimer]

* Rich Felker:

>> this can allow earlier single threaded detection than only
>> considering pthread_join: e.g. stdio, malloc etc may do a
>> check and update the global after an acquire barrier, however
>> the compiler must not cache globals across libc calls for this
>> to work.
>
> It can't cache globals across non-pure functions whose definitions it
> cant't see (and if it saw the definition it would know the global is
> modified).

All 

> malloc is something of a special case where clang treats it
> not as a function but having "pure malloc semantics", but even then I
> don't think it matters if it caches it; at worst you see the old value
> of __libc_single_threaded (false) rather than the new one (true) and
> that direction is safe.
>
> Rich