Update Dartdoc of Function

The Dartdoc of the class `Function` claimed that any class with a
`call` method is a subtype of `Function`. That rule is highly obsolete,
and this PR removes it.

Change-Id: I56318f6c6d8a19007c2b6259bc7d84532cd44707
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/154004
Reviewed-by: Lasse R.H. Nielsen <lrn@google.com>
Commit-Queue: Lasse R.H. Nielsen <lrn@google.com>

Author: eernstg

sdk/lib/core/function.dart

@@ -7,8 +7,8 @@ part of dart.core;
 /**
  * The base class for all function types.
  *
- * A function value, or an instance of a class with a "call" method, is a
- * subtype of a function type, and as such, a subtype of [Function].
+ * The run-time type of a function object is subtype of a function type,
+ * and as such, a subtype of [Function].
  */
 abstract class Function {
   /**