Document scrolltorange event

https://github.com/elifesciences/pattern-library/pull/873 changes eLife to react to the `scrolltorange` event to open collapsed sections rather than trying to open everything when we can detect the Hypothesis client opening (which isn't entirely possible). (This was mentioned in https://github.com/hypothesis/client/issues/660#issuecomment-361976811.)

This event appears to be designed for this use case, but isn't documented anywhere (so we weren't aware of it). Could it be, so we won't have problems relying on it?