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

[Matthias Bussonnier]

Change by Matthias Bussonnier <bussonniermatthias at gmail.com>:


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keywords: +patch
pull_requests: +13177
stage:  -> patch review

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue36896>
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