Documentation: Element's dependencies order

There is no documentation on how dependencies should be ordered.

If the element depends on a set of elements each depending each other, documentation should state if the order of each dependency declaration matters or not.

From this comment: https://gitlab.gnome.org/GNOME/gnome-build-meta/merge_requests/22#note_96808

When it explain, put "Element1" before "Element2", can mean put Element1 up of Element2, so Element1 is build first then Element2.