Fix polynomial_autograd documentation to match implementation

[patrocinio]

The tutorial code uses exp(x) but the documentation incorrectly stated it was learning sin(x). This fix updates the documentation to correctly describe what the code does:

• Changed documentation from sin(x) from -π to π
• To exp(x) from -1 to 1
• Added explanation about Taylor expansion of exponential function

The code remains unchanged and continues to demonstrate fitting a polynomial to the exponential function using PyTorch autograd.

Fixes #3632

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.