atexit.3: ATTRIBUTES: Note function that is thread-safe

The marking matches glibc marking.
The marking of functions in glibc is:
- atexit: MT-Safe

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

man3/atexit.3

@@ -73,6 +73,19 @@ The
 .BR atexit ()
 function returns the value 0 if successful; otherwise
 it returns a nonzero value.
+.SH ATTRIBUTES
+For an explanation of the terms used in this section, see
+.BR attributes (7).
+.TS
+allbox;
+lb lb lb
+l l l.
+Interface	Attribute	Value
+T{
+.BR atexit ()
+T}	Thread safety	MT-Safe
+.TE
+
 .SH CONFORMING TO
 SVr4, 4.3BSD, C89, C99, POSIX.1-2001.
 .SH NOTES