getrlimit.2: ATTRIBUTES: Note functions that are thread-safe

After research, We think prlimit() is thread-safe. But, there
is not marking of prlimit() in glibc document.
getrlimit() and setrlimit() match glibc markings.
- getrlimit: MT-Safe
- setrlimit: MT-Safe
- prlimit:   MT-Safe

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

man2/getrlimit.2

@@ -508,6 +508,21 @@ The
 .BR prlimit ()
 system call is available since Linux 2.6.36.
 Library support is available since glibc 2.13.
+.SH ATTRIBUTES
+For an explanation of the terms used in this section, see
+.BR attributes (7).
+.TS
+allbox;
+lbw35 lb lb
+l l l.
+Interface	Attribute	Value
+T{
+.BR getrlimit (),
+.BR setrlimit (),
+.BR prlimit ()
+T}	Thread safety	MT-Safe
+.TE
+
 .SH CONFORMING TO
 .BR getrlimit (),
 .BR setrlimit ():