[PATCH v2 2/2] manual: Document __libc_single_threaded

[Florian Weimer]

* DJ Delorie:

>> +Atomic operations can also be used on mappings shared among
>> +single-threaded processes.  This means that a compiler cannot use
>> +@code{__libc_single_threaded} to optimize atomic operations, unless it
>> +is able to prove that the memory is not shared.
>
> Ok.  "must not" would be slightly more correct than "cannot" though.

Thanks, applied.

Florian