style.mdoc.5: Suggest preferred formatting for EXAMPLES

Add an example of how to format examples in EXAMPLES sections. The
suggested format is heavily based on zfs.8.

While here, capitalize subsection titles.

Reviewed by: bcr
Approved by: bcr (doc),
Approved by: krion (mentor, implicit), mat (mentor, implicit)
Differential Revision: https://reviews.freebsd.org/D18681