man/depmod.d: add external keyword description
man/depmod.d: add external keyword description

The commit 'depmod: implement external directories support' added
external directories support (see
7da6884e7357ac05772e90f6d7e63b1948103fc4).

This patch documents the extention in the manpage.

Signed-off-by: Yauheni Kaliuta <0d6c66cf06ee77089f9b834ad16e0f050cf87c60@redhat.com>

lucasdemarchi/kmod,lucasdemarchi/kmod,pmmccorm/kmod,lucasdemarchi/kmod,pmmccorm/kmod,pmmccorm/kmod