Add spl_kmem_cache_kmem_threads man page entry

The spl_kmem_cache_kmem_threads module option was accidentally
omitted from the documentation.  Add it.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes openzfs#512

man/man5/spl-module-parameters.5

@@ -41,6 +41,20 @@ concurrently.
 Default value: \fB0x02\fR
 .RE
 
+.sp
+.ne 2
+.na
+\fBspl_kmem_cache_kmem_threads\fR (uint)
+.ad
+.RS 12n
+The number of threads created for the spl_kmem_cache task queue.  This task
+queue is responsible for allocating new slabs for use by the kmem caches.
+For the majority of systems and workloads only a small number of threads are
+required.
+.sp
+Default value: \fB4\fR
+.RE
+
 .sp
 .ne 2
 .na