MFC r363363, r363416: geli(8): new example with file based storage

r363363:
geli(8): Add an example on how to use geli(8) with a file as encrypted storage

Reviewed by: bcr (mentor)
Approved by: bcr (mentor)
Differential Revision: https://reviews.freebsd.org/D25741

r363416:
geli(8): Add missing commands in the EXAMPLES section

Reported by: Fabian Keil <freebsd-listen at fabiankeil dot de>
Reviewed by: bcr (mentor)
Approved by: bcr (mentor)
Differential Revision: https://reviews.freebsd.org/D25761