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

The markings match glibc markings.
markings of functions in glibc are:
- err:    MT-Safe locale
- err:    MT-Safe locale
- errx:   MT-Safe locale
- warn:   MT-Safe locale
- warnx:  MT-Safe locale
- verr:   MT-Safe locale
- verrx:  MT-Safe locale
- vwarn:  MT-Safe locale
- vwarnx: MT-Safe locale

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

man3/err.3

@@ -106,6 +106,29 @@ and
 .BR verrx ()
 functions do not return, but exit with the value of the argument
 .IR eval .
+.SH ATTRIBUTES
+For an explanation of the terms used in this section, see
+.BR attributes (7).
+.TS
+allbox;
+lbw17 lb lb
+l l l.
+Interface	Attribute	Value
+T{
+.BR err (),
+.BR errx (),
+.br
+.BR warn (),
+.BR warnx (),
+.br
+.BR verr (),
+.BR verrx (),
+.br
+.BR vwarn (),
+.BR vwarnx ()
+T}	Thread safety	MT-Safe locale
+.TE
+
 .SH CONFORMING TO
 These functions are nonstandard BSD extensions.
 .\" .SH HISTORY