distance property documented but missing from waypoints

[danpat]

Our docs mention in several places that waypoint objects in responses to pretty much all APIs should contain a distance property.

However, only the nearest plugin actually does this - other APIs (table, route, etc) don't include a distance property for waypoints.

In many of our sample snippets in the docs, the distance property is shown for waypoint objects, even though we don't actually emit one.

We should just be consistent here and return it always. The nearest plugin here:

https://github.com/Project-OSRM/osrm-backend/blob/master/include/engine/api/nearest_api.hpp#L43-L44

shows how the property can be added - although this could be moved to the BaseAPI and just done for all cases.

Once it's consistently available, we should also update the docs.

[danpaz]

Released in 5.21.0