Isochrone intersections lack documentation and api-tests

#### Here's what I did
I tried to write an API-Test for Isochrone requests with `intersections:true` in body.
To do s, I wanted to figure out how isochrone intersections work and what will be returned, as to be able to check for correct returns.

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#### Here's where I got
nothing - isochrone intersections aren't documented. In fact, Isochrone responses aren't documented at all, as far as I could tell.

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#### Here's what I was expecting
Some documentation

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#### Here's what I think could be improved
Isochrone documentation :)