Fix matrix0 documentation reference and compiler warnings

[cxzhong]

Summary

• replace the unresolved Sphinx method reference to the Cython-only _set_to_product hook with literal documentation
• initialize the row and column indices used by Matrix.__getitem__ and Matrix.__setitem__

Root cause

_set_to_product is a cdef method and is not exposed as a Python class member, so Sphinx cannot resolve it as a py:meth target. The uninitialized-index warnings are false positives from the generated C control flow, but they fail to prove to the compiler that the variables are assigned whenever their corresponding flags are set.

Validation

• rebuilt matrix0.pyx with no -Wmaybe-uninitialized warnings
• python -m sage.doctest --warn-long 60 src/sage/matrix/matrix0.pyx (990 tests passed)
• rebuilt reference/matrices HTML documentation successfully; the original unresolved-reference warning is gone

[github-actions]

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[kwankyu]

OK.