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>

@@ -269,8 +269,31 @@
 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)