The attached patch adds a couple of section about the single and multiple clones approaches. The patch is still incomplete, because the rest of the page should be adapted to the new content (in particular the old sections should be removed, and the whole structure revisited), but I wanted some early feedback about the ones I added.

The idea is that contributors can use the single clone approach, and since they don't need to commit/merge, they don't need more instructions then the ones provided already in the setup.rst page.
Committers are better off with multiple clones, and the best way to do it is IMHO with the share extension, so that's what I described.
Ideally this section should be followed by a FAQ-like list that explains how to deal with conflicts, head merges, null merges, long-term features and similar.  Some of these things are already there; some extra things are also there, and they should probably be moved or removed.