rec docs: Document how to disable allow_notify_for

Signed-off-by: Janne Heß <janne@hess.ooo>

Author: dasJ

pdns/recursordist/rec-rust-lib/table.py

@@ -110,6 +110,8 @@
 Domain names specified in this list are used to permit incoming
 NOTIFY operations to wipe any cache entries that match the domain
 name. If this list is empty, all NOTIFY operations will be ignored.
+Adding a single ``.`` to the list disables this check altogether
+and allows clearing all zones.
  ''',
         'versionadded': '4.6.0',
         'runtime': ['reload-acls'],