Document the "Organic Groups" entity selection handler inside the UI [#2692509]

Since three years, OG supports configurable selection handlers in OG reference field instances with the "Entity selection: Mode" setting.

However, it is not apparent from the UI what the different selection handlers do. Took me some hours of trial and error and finally finding documentation coming with the original commit.

If I understand things right, the "Entity selection: Mode" setting is not a OG invention, but provided by the Entity Reference field type. OG "just" provided a new option "Organic Groups" for that setting, and is provided a sub-form for configuring that option. If so, documentation can (and should) be added by OG to that sub-form.

This UI-integrated documentation should esp. include that this selection handler limits options beyond what can be configured in the UI (which is counter-intuitive, so deserves documentation). Namely, it does not allow selecting groups of which the user is not a member of, even if she has a right to create content in such groups. And that the user should select a different selection handler if that behavior is not desired. (The "Simple" and "Commons OG" selection handlers work for me for that purpose.) That is at least my understanding of the resolution currently proposed for #1902086: Allow group-audience widget to allow adding new content to groups a user doesn't belong to.