Improve Java documentation to not refer to Scala code

As discussed in #1298 the Java documentation currently includes Scala code snippets when referring to how the Java DSL is defined. We should improve the Java documentation to not use any Scala code by either linking to the Javadoc for the actual definition, providing Java equivalent definitions or rephrasing the documentation to not mention how the API is defined.

Places can be found with the following git grep:

> git grep '@snip .*/javadsl/.*\.scala'
docs/src/main/paradox/java/http/server-side-https-support.md:@@snip [ConnectionContext.scala](../../../../../../akka-http-core/src/main/scala/akka/http/javadsl/ConnectionContext.scala) { #https-context-creation }
docs/src/main/paradox/java/http/server-side/websocket-support.md:@@snip [Message.scala](../../../../../../../akka-http-core/src/main/scala/akka/http/javadsl/model/ws/Message.scala) { #message-model }
docs/src/main/paradox/scala/http/client-side/client-https-support.md::  @@snip [ConnectionContext.scala](../../../../../../../akka-http-core/src/main/scala/akka/http/javadsl/ConnectionContext.scala) { #https-context-creation }
docs/src/main/paradox/scala/http/client-side/client-transport.md::  @@snip [ClientTransport.scala](../../../../../../../akka-http-core/src/main/scala/akka/http/javadsl/ClientTransport.scala) { #client-transport-definition }