[docs] [issue36896] clarify in types.rst that FunctionTypes & co constructors don't have stable signature

Change by Matthias Bussonnier <bussonniermatthias@gmail.com>: ---------- keywords: +patch pull_requests: +13177 stage: -> patch review _______________________________________ Python tracker <report@bugs.python.org> <https://bugs.python.org/issue36896> _______________________________________