Add note to use QgsApplication::authManager()

python/core/auto_generated/auth/qgsauthmanager.sip.in

@@ -18,6 +18,9 @@ class QgsAuthManager : QObject
 %Docstring
 Singleton offering an interface to manage the authentication configuration database
 and to utilize configurations through various authentication method plugins
+
+QgsAuthManager should not usually be directly created, but rather accessed through
+:py:func:`QgsApplication.authManager()`
 %End
 
 %TypeHeaderCode

src/core/auth/qgsauthmanager.h

@@ -57,6 +57,9 @@ class QTimer;
  * \ingroup core
  * Singleton offering an interface to manage the authentication configuration database
  * and to utilize configurations through various authentication method plugins
+ *
+ * QgsAuthManager should not usually be directly created, but rather accessed through
+ * QgsApplication::authManager().
  */
 class CORE_EXPORT QgsAuthManager : public QObject
 {