4171 clean up spa_feature_*() interfaces
4172 implement extensible_dataset feature for use by other zpool features
Reviewed by: Max Grossman <max.grossman@delphix.com>
Reviewed by: Christopher Siden <christopher.siden@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>

*** 269,276 ****
--- 269,299 ----
  configuration.
  
  When the \fBmulti_vdev_crash_dump\fR feature is set to \fBenabled\fR,
  the administrator can use the \fBdumpadm\fR(1M) command to configure a
  dump device on a pool comprised of multiple vdevs.
+ .RE
  
+ .sp
+ .ne 2
+ .na
+ \fB\fBextensible_dataset\fR\fR
+ .ad
+ .RS 4n
+ .TS
+ l l .
+ GUID    com.delphix:extensible_dataset
+ READ\-ONLY COMPATIBLE   no
+ DEPENDENCIES    none
+ .TE
+ 
+ This feature allows more flexible use of internal ZFS data structures,
+ and exists for other features to depend on.
+ 
+ This feature will be \fBactive\fR when the first dependent feature uses it,
+ and will be returned to the \fBenabled\fR state when all datasets that use
+ this feature are destroyed.
+ 
+ .RE
+ 
  .SH "SEE ALSO"
  \fBzpool\fR(1M)