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The documentation just says that SO_* etc constants are defined. However when people add new ones, they add them as new features to a specific version (not backported as bug fixes), but do not alter the documentation at all. IMO it is silly adding undocumented features. E.g. Issue 26907 was opened to add (among others) SO_PASSCRED, which was already added, undocumented, as part of Issue 6560.
This patch attempts to indicate which symbols are defined by Python (depending on OS availability), and therefore one can deduce if Python does not define a particular symbol. It could be adapted to the 2.7 documentation, but I am not really motivated for that on my own.
I also remove a redundant definition in the module, and removed a conditional because SO_REUSEADDR is required to always be defined according to the test suite. These specific changes should only be applied to 3.6.
I also found Issue 1732367, which has a patch documenting each AF_* symbol in a little more detail. That patch was never updated nor comitted, but it sounds like this kind of addition might be acceptable.