In perimeters.md, replace the reference to the internal
`Graph.isOrthogonal` method with a user-facing explanation:
orthogonal behavior is determined by the `isOrthogonal` metadata
property of the EdgeStyle in the EdgeStyleRegistry.

In edge-styles.md, replace the XXX placeholder in the
EdgeStyleMetaData warning with details about the impact of
`isOrthogonal` on perimeter point computation.

Add cross-links between both pages on this topic.