[SPARK-22855][BUILD] Add -no-java-comments to sbt docs/scalacOptions

Prevents Scala 2.12 scaladoc from blowing up attempting to parse java comments.

## What changes were proposed in this pull request?

Adds -no-java-comments to docs/scalacOptions under Scala 2.12. Also
moves scaladoc configs out of the TestSettings and into the standard sharedSettings
section in SparkBuild.scala.

## How was this patch tested?

SBT_OPTS=-Dscala-2.12 sbt
++2.12.4
tags/publishLocal

Author: Erik LaBianca <[email protected]>

Closes #20042 from easel/scaladoc-212.

Author: easel

project/SparkBuild.scala

@@ -254,6 +254,21 @@ object SparkBuild extends PomBuild {
       "-sourcepath", (baseDirectory in ThisBuild).value.getAbsolutePath  // Required for relative source links in scaladoc
     ),
 
+    // Remove certain packages from Scaladoc
+    scalacOptions in (Compile, doc) := Seq(
+      "-groups",
+      "-skip-packages", Seq(
+        "org.apache.spark.api.python",
+        "org.apache.spark.network",
+        "org.apache.spark.deploy",
+        "org.apache.spark.util.collection"
+      ).mkString(":"),
+      "-doc-title", "Spark " + version.value.replaceAll("-SNAPSHOT", "") + " ScalaDoc"
+    ) ++ {
+      // Do not attempt to scaladoc javadoc comments under 2.12 since it can't handle inner classes
+      if (scalaBinaryVersion.value == "2.12") Seq("-no-java-comments") else Seq.empty
+    },
+
     // Implements -Xfatal-warnings, ignoring deprecation warnings.
     // Code snippet taken from https://issues.scala-lang.org/browse/SI-8410.
     compile in Compile := {
@@ -828,18 +843,7 @@ object TestSettings {
       }
       Seq.empty[File]
     }).value,
-    concurrentRestrictions in Global += Tags.limit(Tags.Test, 1),
-    // Remove certain packages from Scaladoc
-    scalacOptions in (Compile, doc) := Seq(
-      "-groups",
-      "-skip-packages", Seq(
-        "org.apache.spark.api.python",
-        "org.apache.spark.network",
-        "org.apache.spark.deploy",
-        "org.apache.spark.util.collection"
-      ).mkString(":"),
-      "-doc-title", "Spark " + version.value.replaceAll("-SNAPSHOT", "") + " ScalaDoc"
-    )
+    concurrentRestrictions in Global += Tags.limit(Tags.Test, 1)
   )
 
 }