OpenZFS 6738 - zfs send stream padding needs documentation

OpenZFS 6738 - zfs send stream padding needs documentation

Authored by: Eli Rosenthal <eli.rosenthal@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Dan Kimmel <dan.kimmel@delphix.com>
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Reviewed by: Dan McDonald <danmcd@omniti.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>
OpenZFS-issue: https://www.illumos.org/issues/6738
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/c20404ff