MFC r343532:
A few corrections and clarifications to r343406.

• Use "in" instead of "on" when referring to directory and UFS partition.
• Switch from hw.physmem to hw.realmem and add a description to

distinguish the two.

• Explain why the "df" command is having trouble displaying ZFS sizes

correctly. Add a bit more descriptive text to help why the output of
"zfs list -o space" should be used.

• Switch to vmstat instead of iostat display for systat(1) as it shows

more information on one screen. Describe what is displayed based on the
text of the man page. Change the list of the other values accordingly.

• Sort the flags to "zfs destroy" alphabetically.

Reviewed by: rgrimes
Approved by: rgrimes
Differential Revision: https://reviews.freebsd.org/D18993