docs: design: add part-seqnums
docs: design: add part-seqnums

Hopefully clarifies how seqnums should be used and copied from
events to events/messages when those are handled.

https://bugzilla.gnome.org/show_bug.cgi?id=722791

krichter722/gstreamer,ensonic/gstreamer,cablelabs/gstreamer,jpxiong/gstreamer,cfoch/gstreamer,centricular/gstreamer,jpakkane/gstreamer,Lachann/gstreamer,jpakkane/gstreamer,StreamUtils/gstreamer,cfoch/gstreamer,surround-io/gstreamer,jpxiong/gstreamer,krichter722/gstreamer,mparis/gstreamer,centricular/gstreamer,Lachann/gstreamer,mparis/gstreamer,ensonic/gstreamer,jpxiong/gstreamer,jpakkane/gstreamer,StreamUtils/gstreamer,cablelabs/gstreamer,cfoch/gstreamer,centricular/gstreamer,Lachann/gstreamer,cfoch/gstreamer,Lachann/gstreamer,centricular/gstreamer,shelsonjava/gstreamer,jpxiong/gstreamer,ensonic/gstreamer,ensonic/gstreamer,surround-io/gstreamer,mparis/gstreamer,jpakkane/gstreamer,cablelabs/gstreamer,Lachann/gstreamer,surround-io/gstreamer,jpakkane/gstreamer,cablelabs/gstreamer,mparis/gstreamer,jpxiong/gstreamer,cablelabs/gstreamer,ensonic/gstreamer,mparis/gstreamer,surround-io/gstreamer,centricular/gstreamer,krichter722/gstreamer,surround-io/gstreamer,shelsonjava/gstreamer,shelsonjava/gstreamer,krichter722/gstreamer,cfoch/gstreamer,StreamUtils/gstreamer,krichter722/gstreamer,shelsonjava/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,shelsonjava/gstreamer