Improved the documentation for DIB_DNF_MODULE_STREAMS
Improved the documentation for DIB_DNF_MODULE_STREAMS

https://review.opendev.org/c/openstack/diskimage-builder/+/785138
adds the support for DIB_DNF_MODULE_STREAMS which is now available
for all Yum based distros.

This patch enhances the docs for using it for all Yum
based distributions.

Signed-off-by: Chandan Kumar (raukadah) <37614c4f126be5146599b9d19fe2ba37ae0ea495@redhat.com>
Change-Id: I29e726679c2b675b3c0cd95a3ff48fdad7cd5431

openstack/diskimage-builder,openstack/diskimage-builder