zfs mount man page should document legacy behaviour

Document legacy mount behavior.

Reviewed by: Allan Jude <allanjude@freebsd.org>
Reviewed by: Matt Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: bunder2015 <omfgbunder@gmail.com>
Reviewed-by: loli10K <ezomori.nozomu@gmail.com>
Signed-off-by: Kash Pande <kash@tripleback.net>
Closes #2900
Closes #8414