Several of the directive examples in the documentation are underlined red in IntelliJ

This is really confusing for new users which often come from an IDE-only background. We should try to prevent these kind of situations (or make it really explicit in the documentation that this can happen).

It is especially bad, since it also concern the basic examples from here:

http://doc.akka.io/docs/akka-http/current/scala/http/routing-dsl/index.html#minimal-example

Open the Akka HTTP routing DSL minimal-example page and reproduce the examples in IntelliJ. Check which Scala directive examples are underlined red, then make the documentation explicitly explain that this can happen; done means newcomers understand the warning when reading the basic examples.

Written by the indexing model from the issue text.