Updated zfs_dbgmsg_enable documentation to be more accurate

Changed the default specified for zfs_dbgmsg_enable, added
clarification of interaction with zfs_flags.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@ixsystems.com>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes #11984
Closes #11986