docs(linalg): document the argmin/argmax optional-value kernels

[fightBoxing]

Summary

• Document the three public argmin/argmax kernels in lance-linalg that lacked rustdoc: argmax_opt, argmin_value_opt, and argmin_value_float_with_bias.
• Document the bias semantics of argmin_value_float_with_bias: the comparison uses value + bias but the returned value is the original value without the bias term; passing None for bias falls back to argmin_value_float.

Validation

• cargo test -p lance-linalg
• cargo doc -p lance-linalg --no-deps
• cargo fmt --all
• cargo clippy -p lance-linalg --tests -- -D warnings

[lance-gatekeeper]

✅ Gate recommendation: approve.

The rustdoc matches the kernels’ actual boundary behavior, including skipped None values, strict comparisons against the numeric bounds, zip truncation, and biased selection returning the original unbiased value. This is the smallest appropriate change for the documentation gap.