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/os/freebsd/spl/spl_kstat.c

@@ -22,6 +22,10 @@
  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  * SUCH DAMAGE.
+ *
+ * Links to Illumos.org for more information on kstat function:
+ * [1] https://illumos.org/man/1M/kstat
+ * [2] https://illumos.org/man/9f/kstat_create
  */
 
 #include <sys/cdefs.h>

module/os/linux/spl/spl-kstat.c

@@ -22,6 +22,10 @@
  *  with the SPL.  If not, see <http://www.gnu.org/licenses/>.
  *
  *  Solaris Porting Layer (SPL) Kstat Implementation.
+ *
+ *  Links to Illumos.org for more information on kstat function:
+ *  [1] https://illumos.org/man/1M/kstat
+ *  [2] https://illumos.org/man/9f/kstat_create
  */
 
 #include <linux/seq_file.h>