Warn that dbm.dumb.open() can crash Python (GH-6047)

Doc/library/dbm.rst

@@ -361,6 +361,11 @@ The module defines the following:
    database has to be created.  It defaults to octal ``0o666`` (and will be modified
    by the prevailing umask).
 
+   .. warning::
+      It is possible to crash the Python interpreter when loading a database
+      with a sufficiently large/complex entry due to stack depth limitations in
+      Python's AST compiler.
+
    .. versionchanged:: 3.5
       :func:`.open` always creates a new database when the flag has the value
       ``'n'``.