ffs.3: Add a STANDARDS section to the manual page

The ffs() function conforms to IEEE Std 1003.1-2008 ("POSIX.1").
The ffsl() and ffsll() functions conform to IEEE Std 1003.1-2024 ("POSIX.1").

Reviewed by:	ziaee
MFC after:	5 days
Differential Revision:	https://reviews.freebsd.org/D53352

lib/libc/string/ffs.3

@@ -27,7 +27,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd October 17, 2015
+.Dd October 25, 2025
 .Dt FFS 3
 .Os
 .Sh NAME
@@ -80,6 +80,17 @@ argument was zero.
 .Sh SEE ALSO
 .Xr bitstring 3 ,
 .Xr bitset 9
+.Sh STANDARDS
+The
+.Fn ffs
+function conforms to
+.St -p1003.1-2008 .
+The
+.Fn ffsl
+and
+.Fn ffsll
+functions conform to
+.St -p1003.1-2024 .
 .Sh HISTORY
 The
 .Fn ffs