[docs] [issue12067] Doc: remove errors about mixed-type comparisons.

[Guido van Rossum]

Guido van Rossum added the comment:

- This bug should discuss doc updates, not question the rules.

- The rules have evolved over time and the docs stayed behind.

- We should definitely update the 2.7 docs as well as the 3.4 and 3.5 (in development) docs.  The 2.7 docs need to be different than the 3.x docs.

- The language reference manual should clearly state the rules so that implementers can use them as guidelines for implementation.

- There are several sets of relevant rules:

(a) How is each operator translated into a series of lookups and method calls, etc.  It's similar to other binary operators except that the reverse for __lt__ is __gt__ instead of __rlt__, and there's an extra rule that if __ne__ doesn't exist we compute __eq__ and take the opposite.

(b) The default implementation (e.g. default == falls back to 'is', < raises TypeError).

(c) The rules for built-in types, especially numbers (if there are still special cases that aren't explained by the __xx__ methods on the various numeric types).

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nosy: +gvanrossum

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