MFC 358674, 358916: Add examples to ftw.3

ftw.3: Add examples

PR: 173448 [1]
Submitted by: fernape@ (previous version) [1]
Reviewed by: jilles
Approved by: bcr (mentor)
Differential Revision: https://reviews.freebsd.org/D21750

ftw.3: Follow style(9) in the example

Reported by: oshogbo
Approved by: bcr (mentor)
Differential Revision: https://reviews.freebsd.org/D24043