fgetgrent.3: ATTRIBUTES: Note function that is not thread-safe

The marking matches glibc marking.
marking of function in glibc is:
- fgetgrent: MT-Unsafe race:fgrent

ps: We think race:fgrent in glibc maybe hard for users to understand,
and have sent a patch to the GNU libc community for changing it to
race:fgetgrent, however, something about the copyright impeded the progress.
Here we mark it "race:fgetgrent", so there is a little different.

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

man3/fgetgrent.3

@@ -90,6 +90,25 @@ is set to indicate the cause.
 Insufficient memory to allocate
 .I group
 structure.
+.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 fgetgrent ()
+.\" FIXME: the following marking is different from which in glibc manual,
+.\" marking in glibc manual is,
+.\" fgetgrent: MT-Unsafe race:fgrent
+.\" We think race:fgrent in glibc maybe hard for users to understand,
+.\" and have sent a patch to the GNU libc community for changing it to
+.\" race:fgetgrent, however, something about the copyright impeded the progress.
+T}	Thread safety	MT-Unsafe race:fgetgrent
+.TE
+
 .SH CONFORMING TO
 SVr4.
 .SH SEE ALSO