kldconfig(8): Add EXAMPLES to the man page

Add EXAMPLES section to the man page showing the use of all flags except for
-S.

While here, clarify -f description. It not only suppresses diagnostic messages
but it also affects the exit status of the command itself. This is shown in two
of the examples.

Approved by: bcr@
Differential Revision: https://reviews.freebsd.org/D26588