Explain 'zfs list -t snap -o name -s name' speedup

Commit 0cee240 from FreeBSD dramatically speeds up 'zfs list'
performance assuming you're only interested in the dataset
names. This optimization should be mentioned in the man page
to allow end users to take advantage of it.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1847