Expand docs for isCompleted (#104)

Closes #102

This field is confusing because it documents whether the backing
completer has had `complete` called which doesn't necessarily correspond
to when the value is available if it was completed with a `Future`.

lib/src/cancelable_operation.dart

@@ -29,6 +29,11 @@ class CancelableOperation<T> {
   ///
   /// [onCancel] will be called synchronously when the operation is canceled.
   /// It's guaranteed to only be called once.
+  ///
+  /// Calling this constructor is equivalent to creating a [CancelableCompleter]
+  /// and completing it with [inner]. As such, [isCompleted] is true from the
+  /// moment this [CancelableOperation] is created, regardless of whether
+  /// [inner] has completed yet or not.
   factory CancelableOperation.fromFuture(Future<T> inner,
       {FutureOr Function() onCancel}) {
     var completer = CancelableCompleter<T>(onCancel: onCancel);
@@ -125,7 +130,12 @@ class CancelableOperation<T> {
   /// Whether this operation has been canceled before it completed.
   bool get isCanceled => _completer.isCanceled;
 
-  /// Whether this operation completed before being canceled.
+  /// Whether the [CancelableCompleter] backing this operation has been
+  /// completed.
+  ///
+  /// This value being true does not imply that the [value] future has
+  /// completed, but merely that it is no longer possible to [cancel] the
+  /// operation.
   bool get isCompleted => _completer.isCompleted;
 }