OpenZFS 2932 - support crash dumps to raidz, etc. pools

Authored by: Bill Pijewski <wdp@joyent.com>
Reviewed by: Jerry Jelinek <jerry.jelinek@joyent.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Dan McDonald <danmcd@nexenta.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: Giuseppe Di Natale <dinatale2@llnl.gov>

OpenZFS-issue: https://www.illumos.org/issues/2932
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/810e43b
Closes #5984
Closes #5216

man/man5/zpool-features.5

@@ -276,6 +276,34 @@ an existing space map is upgraded to the new format. Once the feature is
 
 .RE
 
+.sp
+.ne 2
+.na
+\fB\fBmulti_vdev_crash_dump\fR\fR
+.ad
+.RS 4n
+.TS
+l l .
+GUID    com.joyent:multi_vdev_crash_dump
+READ\-ONLY COMPATIBLE   no
+DEPENDENCIES    none
+.TE
+
+This feature allows a dump device to be configured with a pool comprised
+of multiple vdevs.  Those vdevs may be arranged in any mirrored or raidz
+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.
+
+Under Linux this feature is registered for compatibility but not used.
+New pools created under Linux will have the feature \fBenabled\fR but
+will never transition to \fB\fBactive\fR.  This functionality is not
+required in order to support crash dumps under Linux.  Existing pools
+where this feature is \fB\fBactive\fR can be imported.
+.RE
+
 .sp
 .ne 2
 .na