[docs] [issue11974] Class definition gotcha.. should this be documented somewhere?

[Raymond Hettinger]

Raymond Hettinger added the comment:

Renee, thanks for the patch.  It looks pretty good.

I've reworked a bit to accomplish the following:

* Added a short introductory a short discussion about class variables versus instance variables.  This addresses other questions that tend to arise irrespective of mutability.

* Referenced the related discussion in"A Word About Names and Objects".

* Referenced the term "mutable" in the glossary.

* change "dangerous" to "unexpected" in accordance with the style guide and following Guido's own wording on the subject.

* Change the class names from "A" and "B" to a "Dog" class because readers tend to find that the more abstract "A" and "B" examples are a little harder to follow.

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Added file: http://bugs.python.org/file35716/class_instance_variables.diff

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