fstat(1): Add EXAMPLES section

• Add examples covering -f, -m and -p flags.

While here, extend the initial description paragraph to note that fstat(1)
will report on all opened files, belonging to processes the user has access to.
The current paragraph may lead to understand that you can get information on
opened files from processes belonging to other users.

Reviewed by: bjk@, danfe@, gbe@
Approved by: manpages (gbe@)
Differential Revision: https://reviews.freebsd.org/D26949