Tweak docs.

doc/site/core/fiber.markdown

@@ -75,7 +75,7 @@ Similar to `Fiber.yield` but provides a value to return to the parent fiber's
 
 ### **call**()
 
-Invokes the fiber or resumes the fiber if it is in a paused state.
+Starts or resumes the fiber if it is in a paused state.
 
     :::dart
     var fiber = new Fiber {
@@ -83,8 +83,23 @@ Invokes the fiber or resumes the fiber if it is in a paused state.
       Fiber.yield()
       IO.print("Fiber called again")
     }
+
+    fiber.call() // Start it.
+    fiber.call() // Resume after the yield() call.
+
+When the called fiber yields, control is transferred back to the fiber that
+called it.
+
+If the called fiber is resuming from a yield, the `yield()` method returns
+`null` in the called fiber.
+
+    :::dart
+    var fiber = new Fiber {
+      IO.print(Fiber.yield())
+    }
+
     fiber.call()
-    fiber.call()
+    fiber.call() // Prints "null".
 
 ### **call**(value)
 
@@ -93,11 +108,11 @@ Invokes the fiber or resumes the fiber if it is in a paused state and sets
 
     :::dart
     var fiber = new Fiber {
-      var value = Fiber.yield()
-      IO.print(value + 1)
+      IO.print(Fiber.yield())
     }
+
     fiber.call()
-    fiber.call(1) // 2.
+    fiber.call("value") // Prints "value".
 
 ### **isDone**