Add missing javadoc interface descriptions for some ActionN variants (#1842)

Author: DavidMGross

src/main/java/rx/functions/Action0.java

@@ -15,6 +15,9 @@
  */
 package rx.functions;
 
+/**
+ * A zero-argument action.
+ */
 public interface Action0 extends Action {
     public void call();
-}
+}

src/main/java/rx/functions/Action1.java

@@ -15,6 +15,9 @@
  */
 package rx.functions;
 
+/**
+ * A one-argument action.
+ */
 public interface Action1<T1> extends Action {
     public void call(T1 t1);
-}
+}

src/main/java/rx/functions/Action2.java

@@ -15,6 +15,9 @@
  */
 package rx.functions;
 
+/**
+ * A two-argument action.
+ */
 public interface Action2<T1, T2> extends Action {
     public void call(T1 t1, T2 t2);
-}
+}

src/main/java/rx/functions/Action3.java

@@ -15,6 +15,9 @@
  */
 package rx.functions;
 
+/**
+ * A three-argument action.
+ */
 public interface Action3<T1, T2, T3> extends Action {
     public void call(T1 t1, T2 t2, T3 t3);
-}
+}