doc: add documentation for USER_SEARCHES

Author: nicoklaus

docs/application-settings.rst

@@ -383,6 +383,42 @@ It must return exactly one result for authentication to succeed.
     AUTH_LDAP_USER_DN="ou=users,dc=example,dc=com"
     AUTH_LDAP_USER_FILTERSTR="(uid=%(user)s)"
 
+USER_SEARCHES
+-------------
+
+To search across multiple LDAP locations, use
+``AUTH_LDAP_USER_SEARCHES``.
+
+When this setting is provided, it overrides both
+``AUTH_LDAP_USER_DN`` and ``AUTH_LDAP_USER_FILTERSTR``.
+
+The setting must contain a JSON list of search definitions. Each entry
+must define:
+
+- ``base``: The base DN to search
+- ``filter``: The LDAP filter (must include ``%(user)s``)
+
+All configured searches are combined using ``LDAPSearchUnion``.
+
+Example:
+
+.. code-block:: python
+
+    AUTH_LDAP_USER_SEARCHES = """
+    [
+        {
+            "base": "ou=users,dc=example,dc=com",
+            "filter": "(uid=%(user)s)"
+        },
+        {
+            "base": "ou=otherusers,dc=example,dc=com",
+            "filter": "(uid=%(user)s)"
+        }
+    ]
+    """
+
+For usage in a ``docker.env`` file only a single line string is accepted.
+
 AUTOCREATE_USER
 ---------------