qsort.3: Document mergesort() behaviour on size overflow

Signed-off-by:	Faraz Vahedi <kfv@kfv.io>
Pull Request:	https://github.com/freebsd/freebsd-src/pull/2243
Reviewed by:	fuz
MFC after:	1 week

lib/libc/stdlib/qsort.3

@@ -29,7 +29,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd October 25, 2024
+.Dd June 2, 2026
 .Dt QSORT 3
 .Os
 .Sh NAME
@@ -362,13 +362,21 @@ functions succeed unless:
 .It Bq Er EINVAL
 The
 .Fa size
-argument is zero, or,
+argument is zero,
 the
 .Fa size
 argument to
 .Fn mergesort
 is less than
-.Dq "sizeof(void *) / 2" .
+.Dq "sizeof(void *) / 2" ,
+or
+the
+.Fa nmemb
+and
+.Fa size
+arguments to
+.Fn mergesort
+describe an unrepresentable buffer size.
 .It Bq Er ENOMEM
 The
 .Fn heapsort