Modifies sentence and provides link to explanation of mutable defaults gotcha in the documentation

[mikejturner]

Summary

Modifies existing sentence and provides link to an explanation of the common gotcha of mutable default parameters for functions/methods. The change occurs on the "Initialization" page (init.html/init.md) of the documentation.

The change adds a link is to this section on python-guide.org. I have no connection to python-guide.org — I just like their explanation and would be happy to change it, if you have a better replacement in mind.

Justification

The documentation already states that mutable default arguments (probably) don't work as expected and alludes to similar gotcha with function/method parameters. I think it could be helpful to provide link to an simple explanation to understand the connection the two situations.

There are many other precedents in the documentation for providing helpful links, for example, "Subclassing considered bad" and "think twice before using pickle"... and this is something that makes me really appreciate attrs documentation.

Pull Request Check List

• Documentation in .rst and .md files is written using semantic newlines.

[hynek]

I'm not sure how I feel about python-guide but that explanation seems good – thanks!