contigmalloc.9: Note that M_WAITOK may still return NULL

Reviewed by:	markj, bapt
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D58382

share/man/man9/contigmalloc.9

@@ -23,7 +23,7 @@
 .\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 .\" POSSIBILITY OF SUCH DAMAGE.
 .\"
-.Dd July 26, 2024
+.Dd July 21, 2026
 .Dt CONTIGMALLOC 9
 .Os
 .Sh NAME
@@ -124,6 +124,15 @@ function returns a kernel virtual address if allocation succeeds,
 or
 .Dv NULL
 otherwise.
+Note that in contrast with
+.Xr malloc 9 ,
+.Fn contigmalloc
+may return
+.Dv NULL
+even if
+.Dv M_WAITOK
+is specified, if no physically congiguous range is available that meets the
+specified constraints.
 .Sh EXAMPLES
 .Bd -literal
 void *p;