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

[Terry J. Reedy]

Terry J. Reedy <tjreedy at udel.edu> added the comment:

In msg342227 Pablo Galindo Salgado said "I am +1 to such a sentence, but I think this is a decision that more core devs should agree on."

> "These types are not supposed to be instantiated outside of CPython internals"

At least Petr Vidtorin and I disagree with this part.  As Petr wrote on pydev thread "Expected stability of PyCode_New() and types.CodeType() signatures", there are multiple tools that instantiate code objects, in particular Cython, which is far from being a rogue project.  Python is a 'consenting adults' languages, and we generally do not officially tell people what they are 'supposed' to do or not do.

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nosy: +petr.viktorin, terry.reedy

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