Add clarifying notes to polynomial tutorials about fit limitations

[patrocinio]

Addresses feedback that the 3rd-degree polynomial fit for sin(x) over [-π, π] may be confusing for beginners. Added educational notes explaining that:

• The example demonstrates gradient descent mechanics, not perfect fitting
• 3rd-degree polynomials have mathematical limitations for this task
• Higher-order terms (5th, 7th degree) are needed for better accuracy
• The imperfect fit is expected and teaches model architecture selection

Fixes #3607

Description

Checklist

• The issue that is being fixed is referred in the description (see above "Fixes #ISSUE_NUMBER")
• Only one issue is addressed in this pull request
• Labels from the issue that this PR is fixing are added to this pull request
• No unnecessary issues are included into this pull request.

[pytorch-bot]

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