Add note about ValueTask and ValueTask<TResult> default constructors (dotnet#4397) (dotnet#4576)

* Add note about ValueTask and ValueTask<TResult> default constructors (dotnet#4397)

* Apply suggestions from code review

Co-authored-by: glihm <[email protected]>
Co-authored-by: Adam Sitnik <[email protected]>

Author: 3 people

xml/System.Threading.Tasks/ValueTask.xml

@@ -64,6 +64,10 @@ Using a `ValueTask` instead of a <xref:System.Threading.Tasks.Task> introduces s
 
 > [!NOTE]
 > The use of the `ValueTask` type is supported starting with C# 7.0 and is not supported by any version of Visual Basic.
+
+> [!NOTE]
+> An instance created with the parameterless constructor or by the `default(ValueTask)` syntax (a zero-initialized structure) represents a synchronously, successfully completed operation.
+
  ]]></format>
     </remarks>
   </Docs>

xml/System.Threading.Tasks/ValueTask`1.xml

@@ -77,6 +77,10 @@ As such, the default choice for any asynchronous method should be to return a <x
 
 > [!NOTE]
 >  The use of the <xref:System.Threading.Tasks.ValueTask%601> type is supported starting with C# 7.0, and is not supported by any version of Visual Basic.
+
+> [!NOTE]
+> An instance created with the parameterless constructor or by the `default(ValueTask<TResult>)` syntax (a zero-initialized structure) represents a synchronously, successfully completed operation with a result of `default(TResult)`.
+
  ]]></format>
     </remarks>
   </Docs>