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

The marking matches glibc marking.

Signed-off-by: Ma Shimiao <mashimiao.fnst@cn.fujitsu.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man3/wctob.3

@@ -47,6 +47,19 @@ function returns the single-byte representation of
 if it exists, of
 .B EOF
 otherwise.
+.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 wctob ()
+T}	Thread safety	MT-Safe
+.TE
+
 .SH CONFORMING TO
 C99.
 .SH NOTES