df(1): Add EXAMPLES section to man page

• Add EXAMPLES section with four simple examples.
• Simplify -H flag description. This makes easy to see the difference between this flag and -h
• While here, fix .Tn deprecated macro.

Approved by: manpages (bcr@)
Differential Revision: https://reviews.freebsd.org/D26662

(cherry picked from commit ad3d2e7e4a18575f825b65a19e49720a513d2343)