gh-37615: Removed `BUG` method and include a docstring for `matrix_of_frobenius()`
When reviewing code for hyperelliptic curves I found a method labeled
bug with a typo in the name which was unnecessary. I have removed this
method and included doctests which were previously absent.
### :memo: Checklist
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- [x] The title is concise and informative.
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- [x] I have created tests covering the changes.
- [x] I have updated the documentation accordingly.
URL: https://github.com/sagemath/sage/pull/37615
Reported by: Giacomo Pope
Reviewer(s): Giacomo Pope, grhkm21
