docs: strip task-tracking IDs from code comments and fill missing XML docs

Tracking IDs are review bookkeeping that rots in shipped source; the rationale they tagged is kept. Comment-only change (CommentChecker 458 -> 16 false positives).

clients/dotnet/ZB.MOM.WW.MxGateway.Client/MxGatewaySession.cs

@@ -854,6 +854,7 @@ public sealed class MxGatewaySession : IAsyncDisposable
     /// </summary>
     /// <param name="serverHandle">The ServerHandle from register.</param>
     /// <param name="cancellationToken">Cancellation token.</param>
+    /// <returns>A task that represents the asynchronous operation.</returns>
     public async Task UnregisterAsync(
         int serverHandle,
         CancellationToken cancellationToken = default)
@@ -888,6 +889,7 @@ public sealed class MxGatewaySession : IAsyncDisposable
     /// <param name="serverHandle">The ServerHandle from register.</param>
     /// <param name="itemHandle">The ItemHandle from add-item.</param>
     /// <param name="cancellationToken">Cancellation token.</param>
+    /// <returns>A task that represents the asynchronous operation.</returns>
     public async Task AdviseSupervisoryAsync(
         int serverHandle,
         int itemHandle,
@@ -985,6 +987,7 @@ public sealed class MxGatewaySession : IAsyncDisposable
     /// <param name="serverHandle">The ServerHandle from register.</param>
     /// <param name="updateIntervalMilliseconds">The buffered update interval, in milliseconds.</param>
     /// <param name="cancellationToken">Cancellation token.</param>
+    /// <returns>A task that represents the asynchronous operation.</returns>
     public async Task SetBufferedUpdateIntervalAsync(
         int serverHandle,
         int updateIntervalMilliseconds,
@@ -1125,6 +1128,7 @@ public sealed class MxGatewaySession : IAsyncDisposable
     /// <param name="currentUserId">The current operator user id.</param>
     /// <param name="verifierUserId">The verifier (secondary approver) user id.</param>
     /// <param name="cancellationToken">Cancellation token.</param>
+    /// <returns>A task that represents the asynchronous operation.</returns>
     public async Task WriteSecuredAsync(
         int serverHandle,
         int itemHandle,
@@ -1204,6 +1208,7 @@ public sealed class MxGatewaySession : IAsyncDisposable
     /// <param name="currentUserId">The current operator user id.</param>
     /// <param name="verifierUserId">The verifier (secondary approver) user id.</param>
     /// <param name="cancellationToken">Cancellation token.</param>
+    /// <returns>A task that represents the asynchronous operation.</returns>
     public async Task WriteSecured2Async(
         int serverHandle,
         int itemHandle,