docs: override is not the same as overwrite!

docs: override is not the same as overwrite!

Some docs claimed that values in certain config files would be
overwritten, when they would actually be overridden.

Override: a file is not modified but its contents are superseded by
          something else.  (configurations set in
          /etc/dracut.conf.d/*.conf override configurations set in
          /etc/dracut.conf)

Overwrite: a file is modified or its contents replaced by an action
           (use dracut --force to overwrite the existing initramfs)