Fix CompletedTaskAttribute preprocessor directive and update documentation

Co-authored-by: drewnoakes <[email protected]>

Author: Copilot

docfx/analyzers/VSTHRD003.md

@@ -10,6 +10,32 @@ When required to await a task that was started earlier, start it within a delega
 `JoinableTaskFactory.RunAsync`, storing the resulting `JoinableTask` in a field or variable.
 You can safely await the `JoinableTask` later.
 
+## Suppressing warnings for completed tasks
+
+If you have a property, method, or field that returns a pre-completed task (such as a cached task with a known value),
+you can suppress this warning by applying the `[CompletedTask]` attribute to the member.
+This attribute is automatically included when you install the `Microsoft.VisualStudio.Threading.Analyzers` package.
+
+```csharp
+[Microsoft.VisualStudio.Threading.CompletedTask]
+private static readonly Task<bool> TrueTask = Task.FromResult(true);
+
+async Task MyMethodAsync()
+{
+    await TrueTask; // No warning - TrueTask is marked as a completed task
+}
+```
+
+The analyzer already recognizes the following as safe to await without the attribute:
+- `Task.CompletedTask`
+- `Task.FromResult(...)`
+- `Task.FromCanceled(...)`
+- `Task.FromException(...)`
+- `TplExtensions.CompletedTask`
+- `TplExtensions.CanceledTask`
+- `TplExtensions.TrueTask`
+- `TplExtensions.FalseTask`
+
 ## Simple examples of patterns that are flagged by this analyzer
 
 The following example would likely deadlock if `MyMethod` were called on the main thread,

src/Microsoft.VisualStudio.Threading.Analyzers.CodeFixes/buildTransitive/AdditionalFiles/CompletedTaskAttribute.cs

@@ -2,6 +2,7 @@
 // Licensed under the MIT license. See LICENSE file in the project root for full license information.
 
 #if !COMPLETEDTASKATTRIBUTE_INCLUDED
+#define COMPLETEDTASKATTRIBUTE_INCLUDED
 
 namespace Microsoft.VisualStudio.Threading;
 
@@ -16,18 +17,10 @@ namespace Microsoft.VisualStudio.Threading;
 /// as awaiting an already-completed task does not pose a risk of deadlock.
 /// </remarks>
 [System.AttributeUsage(System.AttributeTargets.Property | System.AttributeTargets.Method | System.AttributeTargets.Field, Inherited = false, AllowMultiple = false)]
-internal sealed class CompletedTaskAttribute : System.Attribute
-{
-}
-
-#pragma warning disable SA1403 // File may only contain a single namespace
 #pragma warning disable SA1649 // File name should match first type name
-internal static class CompletedTaskAttributeDefinition
+internal sealed class CompletedTaskAttribute : System.Attribute
 {
-    internal const bool Included = true;
 }
 #pragma warning restore SA1649 // File name should match first type name
-#pragma warning restore SA1403 // File may only contain a single namespace
 
-#define COMPLETEDTASKATTRIBUTE_INCLUDED
 #endif