Trivial changes: improved documentations for __contains__

[alkeldi]

The previous strings for list.__contains__.__doc__ and tuple.__contains__.__doc__ were "Return key in self.". This is misleading, because __contains__ returns a boolean.

I modified these strings and the string in set.__contains__.__doc__ to a semilar format of that in dict.__contains__.__doc__.

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[rhettinger]

How is the current wording misleading? I don't follow what you're saying about returning a boolean.

Also, I think we're losing something important if the docstrings for dunder methods stops telling users about the associated operators, __eq__(self, other) called by self==value and __contains__(self, key) called by key in self.

[alkeldi]

I think the misleading part is Return key in self because it suggests returning a key that exists in self. However, the returned value is the boolean result of the in operator. It's not clear that in is meant to be the operator. What's happening here is basically documenting code with another code, but that's not clear.

As for the other modification, which is in the set-object, it's just to make the docstrings for all the basic datatypes similar.

Another way to do this, is making the docstrings =x.__contains__(y) <==> y in x for all the datatypes. In this context it's easier to see that in is the operator.

Also, dict has __contains__.__doc__ = True if the dictionary has the specified key, else False., which doesn't mention anything about the associated operator.

[rhettinger]

I'm going to unassign from this one. I don't really buy into the notion that people are going to misread the docstring and believe that d.__contains__(key) is going to "return some other key from the dict". I've never encountered anyone who read it that way.