Add help message for inspect actual. Fixes #1651.

main/src/main/scala/sbt/CommandStrings.scala

@@ -124,6 +124,12 @@ $ShowCommand <task>
     |$InspectCommand definitions <key>
     |
     |	Displays the scopes in which `key` is defined.
+    |
+    |$InspectCommand actual <key>
+    |
+    |	Displays the actual dependencies used by `key`.
+    |	This is useful because delegation means that a dependency can come from a scope other than the requested one.
+    |	Using `inspect actual` will show exactly which scope is providing a value for a setting.
   """.stripMargin.trim
 
   val SetCommand = "set"

notes/0.13.9/help-inspect-actual.markdown

@@ -0,0 +1,12 @@
+
+  [@dwijnand]: http://github.com/dwijnand
+  [1651]: https://github.com/sbt/sbt/issues/1651
+  [1990]: https://github.com/sbt/sbt/pull/1990
+
+### Fixes with compatibility implications
+
+### Improvements
+
+- Add help message for `inspect actual`. [#1651][1651]/[#1990][1990] by [@dwijnand][@dwijnand]
+
+### Bug fixes