docs(admin): warn about Configuration Active before LDAP testing

Add a warning block before the "Testing the configuration" section to
highlight that Configuration Active must be enabled under the Advanced
tab before testing. Without it authentication fails with a misleading
"wrong password" error even when all other settings are correct.

Fixes #912

Signed-off-by: skjnldsv <skjnldsv@protonmail.com>

Author: skjnldsv

admin_manual/configuration_user/user_auth_ldap.rst

@@ -630,6 +630,12 @@ Username-LDAP User Mapping
 Testing the configuration
 -------------------------
 
+.. warning:: Before testing, make sure the **Configuration Active** checkbox is
+   enabled under the **Advanced** tab. It is disabled by default and will cause
+   authentication to fail with a "wrong password" error even if all other settings
+   are correct. Nextcloud only enables it automatically after a successful test
+   connection.
+
 The **Test Configuration** button checks the values as currently given in the
 input fields. You do not need to save before testing. By clicking on the
 button, Nextcloud will try to bind to the Nextcloud server using the