Add xmldoc explaining the array response workaround

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: barnon-apiiro

src/GitLabApiClient/WebhookClient.cs

@@ -40,7 +40,9 @@ public async Task<IList<Webhook>> GetAsync(ProjectId projectId)
         }
 
         /// <summary>
-        /// Create new webhook
+        /// Create new webhook.
+        /// Some GitLab versions return a JSON array from POST /projects/:id/hooks
+        /// instead of a single object. We deserialize as JToken to handle both formats.
         /// </summary>
         /// <param name="projectId">The ID, path or <see cref="Project"/> of the project.</param>
         /// <param name="request">Create hook request.</param>