Fix scala-js/scala-js#1917: Add -Xfatal-warnings for ScalaDoc generation

Note that links to the JavaDoc are not yet correct (filed as scala-js/scala-js#1970).

Author: gzm0

js-envs/src/main/scala/org/scalajs/jsenv/AsyncJSRunner.scala

@@ -47,7 +47,7 @@ trait AsyncJSRunner {
   final def await(): Unit = Await.result(future, Duration.Inf)
 
   /** Await completion of the started Run for the duration specified
-   *  by [[atMost]]. Strictly equivalent to:
+   *  by `atMost`. Strictly equivalent to:
    *
    *  {{{
    *  Await.result(future, atMost)
@@ -57,7 +57,7 @@ trait AsyncJSRunner {
   final def await(atMost: Duration): Unit = Await.result(future, atMost)
 
   /** Awaits completion of the started Run for the duration specified by
-   *  [[atMost]], or force it to stop.
+   *  `atMost`, or force it to stop.
    *
    *  If any exception is thrown while awaiting completion (including a
    *  [[scala.concurrent.TimeoutException TimeoutException]], forces the runner

js-envs/src/main/scala/org/scalajs/jsenv/ComJSRunner.scala

@@ -9,15 +9,15 @@ trait ComJSRunner extends AsyncJSRunner {
 
   /** Blocks until a message is received and returns it.
    *
-   *  @throws ComClosedException if the channel is closed before a message
-   *    is received
+   *  @throws ComJSEnv.ComClosedException if the channel is closed before a
+   *    message is received
    */
   final def receive(): String = receive(Duration.Inf)
 
   /** Blocks until a message is received and returns it.
    *
-   *  @throws ComClosedException if the channel is closed before a message
-   *    is received
+   *  @throws ComJSEnv.ComClosedException if the channel is closed before a
+   *    message is received
    *  @throws scala.concurrent.TimeoutException if the timeout expires
    *    before a message is received and the channel is still open
    */

js-envs/src/main/scala/org/scalajs/jsenv/ExternalJSEnv.scala

@@ -84,7 +84,7 @@ abstract class ExternalJSEnv(
 
     /** Wait for the VM to terminate, verify exit code
      *
-     *  @throws NonZeroExitException if VM returned a non-zero code
+     *  @throws ExternalJSEnv.NonZeroExitException if VM returned a non-zero code
      */
     final protected def waitForVM(vmInst: Process): Unit = {
       // Make sure we are done.