Add note about not exposing fine-grained autoplugin inclusion controlls.

AddSettings should only expose coarse-grained features of AutoPlugins
or else the Logic we use to ensure safe addition completely breaks
down.  Leaving it in the code as an escape hatch if we get desparate,
but we need an alternative for controlling ordering later.

main/src/main/scala/sbt/AddSettings.scala

@@ -19,7 +19,12 @@ object AddSettings
 	private[sbt] final object ProjectSettings extends AddSettings
 
 	/** Adds all settings from autoplugins. */
-	val autoPlugins: AddSettings = new AutoPlugins(const(true))
+	val autoPlugins: AddSettings = new AutoPlugins(const(true))  // Note: We do not expose fine-grained autoplugins because
+	                                                             // it's dangerous to control at that level right now.
+	                                                             // Leaving the hook in place in case we need to expose
+	                                                             // it, but most likely it will remain locked out
+	                                                             // for users with an alternative ordering feature
+	                                                             // in place.
 
 	/** Settings specified in Build.scala `Project` constructors. */
 	val projectSettings: AddSettings = ProjectSettings