Add missing documentation for some KMC flags

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Matt Ahrens <matt@delphix.com>
Signed-off-by: Sebastian Gottschall <s.gottschall@dd-wrt.com>
Signed-off-by: Michael Niewöhner <foss@mniewoehner.de>
Closes #9034

module/os/linux/spl/spl-kmem-cache.c

@@ -876,11 +876,11 @@ spl_magazine_destroy(spl_kmem_cache_t *skc)
  *	KMC_VMEM        Force SPL vmem backed cache
  *	KMC_SLAB        Force Linux slab backed cache
  *	KMC_OFFSLAB	Locate objects off the slab
- *	KMC_NOTOUCH	unsupported
- *	KMC_NODEBUG	unsupported
- *	KMC_NOHASH      unsupported
- *	KMC_QCACHE	unsupported
- *	KMC_NOMAGAZINE	unsupported
+ *	KMC_NOTOUCH	Disable cache object aging (unsupported)
+ *	KMC_NODEBUG	Disable debugging (unsupported)
+ *	KMC_NOHASH      Disable hashing (unsupported)
+ *	KMC_QCACHE	Disable qcache (unsupported)
+ *	KMC_NOMAGAZINE	Enabled for kmem/vmem, Disabled for Linux slab
  */
 spl_kmem_cache_t *
 spl_kmem_cache_create(char *name, size_t size, size_t align,