[#1336] Describe guides for default arguments.


Problem description
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Mutable default arguments are initialized using None and then assigned an default value.

In this case it is acceptable to re-write input arguments... since they don't really exist.

Changes description
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Add initial text to describe this pattern.


How to try and test the changes
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reviewers @alibotean @bnickname 

Check that changes make sense.
Feel free to add more comments and improve the code.