fix(adjoint): update docstrings for DerivativeInfo fields

[groberts-flex]

thanks @marcorudolphflex for catching that we had these stale docstrings! Updated here based on recent shape gradient overhaul!

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Note

Low Risk
Docstring/type-hint-only changes plus a changelog entry; no functional gradient computation logic is modified.

Overview
Updates DerivativeInfo documentation in derivative_utils.py to match the post-shape-gradient-overhaul semantics: eps_in is now documented/typed as optional (EpsType | None), eps_out and inside/outside meanings are clarified, and create_interpolators() docstrings are updated to reflect the current permittivity/interpolator set (including optional H-field interpolators).

Adds a corresponding Unreleased changelog note about the DerivativeInfo docstring corrections.

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[github-actions]

Diff Coverage

Diff: origin/develop...HEAD, staged and unstaged changes

• tidy3d/components/autograd/derivative_utils.py (100%)

Summary

• Total: 1 line
• Missing: 0 lines
• Coverage: 100%