Add more guidance to more refined title/creator metadata

[AndersEkl]

Originally written by @martinpub

I'ts tempting to just don't do any extra validation on the ordering of the dc:titles (and maybe not even title-type?), as this is left for the ordering agencies to specify in their own markup instructions. But I think maybe the guidelines should mention the display-seq attribute together with the title-type?

Originally posted by @josteinaj in https://github.com/nlbdev/epub3-guidelines-update/issues/86#issuecomment-795413089

[oscarlcarlsson]

Looking at 5.5.3.2 The dc:title element in the epub3.3 specifications, display-seq is not well supported. Should we still consider adding this?

Previous versions of this specification recommended using the title-type and display-seq properties to identify and format the segments of multipart titles (see the Great Cookbooks example). It is still possible to add these semantics, but they are also not well supported.

[martinpub]

Good catch. It's prio 3 so I don't think we should dwell on it. It is still possible to use it, but will likely not be used in reading systems any time soon. Let's keep the issue open though?