uname.1: Document which flags are non-POSIX

MFC after:		3 days
Reviewed by:		ziaee
Differential Revision:	https://reviews.freebsd.org/D58476

usr.bin/uname/uname.1

@@ -25,7 +25,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd September 14, 2022
+.Dd July 28, 2026
 .Dt UNAME 1
 .Os
 .Sh NAME
@@ -149,6 +149,16 @@ The
 command is expected to conform to the
 .St -p1003.2
 specification.
+.Pp
+The
+.Fl b ,
+.Fl i ,
+.Fl K ,
+.Fl o ,
+.Fl p ,
+and
+.Fl U
+options are non-standard extensions.
 .Sh HISTORY
 The
 .Nm