[COMMITED][v2] manual: Remove incorrect claim that qsort() can be stabilized

[Paul Eggert]

Ondřej Bílka wrote:
> +The addresses passed to the comparison function need not correspond with
> +the original location of the objects, and need not even lie within the
> +original array.

The last clause is incorrect.  C11 and POSIX both require that the addresses 
passed to the comparison function must point to elements of the original array, 
and glibc qsort conforms to the standards here.  Please fix this.