MFC r297509: MFV r297506: 6738 zfs send stream padding needs documentation

MFC r297509: MFV r297506: 6738 zfs send stream padding needs documentation

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>
Author: Eli Rosenthal <eli.rosenthal@delphix.com>

illumos/illumos-gate@c20404ff77119516354b0d112d28b7ea0dadd303

git-svn-id: svn://svn.freebsd.org/base/stable/10@299432 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f