qsort.3: ATTRIBUTES: Note functions that are thread-safe

qsort() matches glibc marking.
>From research, We think qsort_r() is thread-safe. But, there
is not marking of qsort_r() in glibc document.
- qsort:   MT-Safe
- qsort_r: MT-Safe

Signed-off-by: Zeng Linggang <zenglg.jy@cn.fujitsu.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man3/qsort.3

@@ -97,6 +97,20 @@ functions return no value.
 .SH VERSIONS
 .BR qsort_r ()
 was added to glibc in version 2.8.
+.SH ATTRIBUTES
+For an explanation of the terms used in this section, see
+.BR attributes (7).
+.TS
+allbox;
+lbw18 lb lb
+l l l.
+Interface	Attribute	Value
+T{
+.BR qsort (),
+.BR qsort_r ()
+T}	Thread safety	MT-Safe
+.TE
+
 .SH CONFORMING TO
 The
 .BR qsort ()