docs(optimize): document post-hoc convergence check

[lil-lon]

Summary

Closes #542 with the docs-only approach as suggested.

• Added a Notes section to the optimize docstring describing the behavior where final_state is returned even when max_steps is reached, and documented the post-hoc check
• Mirrored the same explanation in the high-level tutorial's Geometry Optimization section

I also checked that the documents are properly rendered locally.

Checklist

Before a pull request can be merged, the following items must be checked:

• Doc strings have been added in the Google docstring format.
• Run ruff on your code.
• Tests have been added for any new functionality or bug fixes.