Clarify radii and distance types in documentation

Clarify radii and distance types in documentation

Because not all circles are round:
* circles using an euclidean metric are what we usually call "round"
* circles using a maximum metric look like euclidean rectangles with equal adjacent sides (squares)
* circles using a manhattan metric look like an euclidean right angled rhombus (squares, but 45° rotated to the former one)

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