Add XML documentation for EnvelopeQuery properties

This commit introduces XML documentation comments for the `UserId`, `Username`, and `UserEmail` properties in the `EnvelopeQuery` record. The comments clarify the purpose of each property, enhancing code readability and maintainability.

EnvelopeGenerator.Application/Envelopes/EnvelopeQuery.cs

@@ -15,9 +15,18 @@ public record EnvelopeQuery(
     int? Status = null,
     string? Uuid = null) : IRequest
 {
+    /// <summary>
+    /// Die eindeutige Kennung des Benutzers.
+    /// </summary>
     public int? UserId => User?.Id;
 
+    /// <summary>
+    /// Der Benutzername des Absenders.
+    /// </summary>
     public string? Username => User?.Username;
 
+    /// <summary>
+    /// Die E-Mail-Adresse des Benutzers.
+    /// </summary>
     public string? UserEmail => User?.Username;
 };