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

[Pablo Galindo Salgado]

Pablo Galindo Salgado <pablogsal at gmail.com> added the comment:

Being said that, I am very happy with the current changes on the PR :)

 Thank you @Terry and @Petr for helping with this!

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<https://bugs.python.org/issue36896>
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