Links in Source Files

Added comments in following files
with links to Illumos manual pages:

./module/avl/avl.c
./module/nvpair/nvpair.c
./module/os/linux/spl/spl-kstat.c
./module/os/freebsd/spl/spl_kstat.c

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Spencer Kinny <spencerkinny1995@gmail.com>
Closes #5113 
Closes #10859

module/avl/avl.c

@@ -96,6 +96,9 @@
  * which each have their own compilation environments and subsequent
  * requirements. Each of these environments must be considered when adding
  * dependencies from avl.c.
+ *
+ * Link to Illumos.org for more information on avl function:
+ * [1] https://illumos.org/man/9f/avl
  */
 
 #include <sys/types.h>