EVENTHANDLER.9: EVENTHANDLER_REGISTER never fails

Since ecdf4409f9 ("Rework the eventhandler locking [...]"),
EVENTHANDLER_REGISTER() can never return NULL.

Suggested by:	olce
Reviewed by:	olce, ziaee, zlei
Approved by:	olce, zlei
Fixes:	ecdf4409f9 ("Rework the eventhandler locking [...]")
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D55014

share/man/man9/EVENTHANDLER.9

@@ -22,7 +22,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd October 7, 2022
+.Dd January 31, 2025
 .Dt EVENTHANDLER 9
 .Os
 .Sh NAME
@@ -125,7 +125,6 @@ as its first parameter along with any additional parameters passed in
 via macro
 .Fn EVENTHANDLER_INVOKE
 (see below).
-If registration is successful,
 .Fn EVENTHANDLER_REGISTER
 returns a cookie of type
 .Vt eventhandler_tag .