Crossbuilding SBT example is misleading

[threeseed]

On this page: https://www.scala-js.org/doc/project/cross-build.html

Below this line: "This is an example how your build.sbt could look like:"

Is an SBT example that would never actually work since it is missing the enablePlugins(ScalaJSPlugin) line. It would be good to add it in.

[sjrd]

The example is correct. In a crossProject, you must not use enablePlugins(ScalaJSPlugin), as it would then also apply on the JVM project. crossProject enables the plugin automatically, and only on the JS part.

[nadenf]

Can we reopen this and add to that page that you should not use enablePlugins for crossProjects.

[sjrd]

#394