Man page updates for 'zfs share'

• Remove the references to share(1M), unshare(1M) and dfstab(4) since they are not applicable to Linux.
• Add the exact exportfs command line used when setting sharenfs=on.

Signed-off-by: Turbo Fredriksson <turbo@bayour.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue: #1641