Document that avoiding tunnels is unsupported

[1ec5]

RouteOptions.roadClassesToAvoid accepts any value in RoadClass, including tunnel, but the Directions API doesn’t support excluding tunnels in its exclude parameter.

Originally, in #154, the assumption was that intersections.classes would only ever indicate classes that the request could exclude. Project-OSRM/osrm-backend#4812 broke this assumption, adding tunnel as a class but not an excludable class. #237 effectively added it as both.

We’ll eventually need to decouple the exclude parameter from the concept of road classes, due to changes like #662, but we can’t formally do that until v3.0.0 due to backwards compatibility concerns. For the time being, we’ll have to settle for updating this documentation comment to note that the option can only be used in the context of Intersection.outletRoadClasses, not RouteOptions.roadClassesToAllow or roadClassesToAvoid:

mapbox-directions-swift/Sources/MapboxDirections/RoadClasses.swift

Lines 49 to 54 in 2db7fda

	/**
	The user must travel this segment of the route through a [tunnel](https://wiki.openstreetmap.org/wiki/Key:tunnel).
	This option can only be used with `RouteOptions.roadClassesToAvoid`.
	*/
	public static let tunnel = RoadClasses(rawValue: 1 << 5)

We could also actively omit tunnel when converting RouteOptions to a request URL, but it doesn’t look like we do that for any other option that’s supported by only one of RouteOptions.roadClassesToAllow or roadClassesToAvoid. In general, we try to avoid client-side enforcement of server-side constraints, in case the Directions API relaxes those constraints in a future deployment or conditionally by user.

/ref mapbox/mapbox-java#709

[azarovalex]

Will be fixed as part of this PR: #664

However, I decided to hardcode supported avoid and include road classes in the separate internal arrays, to filter omit unsupported road classes while creating a URL request.

[azarovalex]

The PR with documentation update was merged