Fix #166 - Add docs for validating default values

[stinovlas]

Closes #166.

Add documentation for how the validating default default values in BaseSettings differs from those in pydantic.BaseModel.

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (3205f81) 97.45% compared to head (e5c06d5) 97.45%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #168   +/-   ##
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  Coverage   97.45%   97.45%           
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  Files           5        5           
  Lines         314      314           
  Branches       69       69           
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  Hits          306      306           
  Misses          6        6           
  Partials        2        2           

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[hramezani]

Thanks @stinovlas for this PR 🙏

I've just simplified the examples.

[stinovlas]

Thanks @stinovlas for this PR 🙏

I've just simplified the examples.

I think the new examples do illustrate the technical aspect of not validating the default value, but lack the illustration of why would you do such a thing. I think my original examples illustrate that quite well and I took care to select example from Python standard library that exists in all Python versions supported by pydantic.

But if you think that the technical aspect is the most important and illustration of the issue is unnecessary, I can't force that on you.