Add XML documentation for GetReferenceTypes method

This commit introduces XML documentation comments to the
GetReferenceTypes method in the HistoryController class.
The comments clarify the method's purpose, indicating that
it retrieves available reference types and returns them
as key-value pairs, along with a summary and return type
information.

EnvelopeGenerator.GeneratorAPI/Controllers/EnvelopeController.cs

@@ -44,7 +44,7 @@ public class EnvelopeController : ControllerBase
         try
         {
             if (User.GetId() is int intId)
-                return await _envelopeService.ReadByUserAsync(intId, min_status: status.Min, max_status: status.Max, ignore_statuses: status.Ignore).ThenAsync(
+                return await _envelopeService.ReadByUserAsync(intId, min_status: status.Min, max_status: status.Max, ignore_statuses: status.Ignore ?? Array.Empty<int>()).ThenAsync(
                     Success: Ok,
                     Fail: IActionResult (msg, ntc) =>
                     {

EnvelopeGenerator.GeneratorAPI/Controllers/HistoryController.cs

@@ -30,6 +30,10 @@ public class HistoryController : ControllerBase
         _service = service;
     }
 
+    /// <summary>
+    /// Ruft die verfügbaren Referenztypen ab und gibt sie als Schlüssel-Wert-Paare zurück.
+    /// </summary>
+    /// <returns>Ein Ok-Ergebnis mit einem Wörterbuch, das die Referenztypen als Schlüssel-Wert-Paare enthält.</returns>
     [HttpGet("reference-type")]
     [Authorize]
     public IActionResult GetReferenceTypes()