Clarify signed-zero behavior in Signed predicate documentation

[RanaPriyansh]

Signed::is_positive and Signed::is_negative document zero as neither positive nor negative. For f32 and f64, the existing implementations instead inspect the sign bit, including signed zero.

Distinguish integer and floating-point behavior in the trait documentation. Add doctests that call Signed explicitly for both float types and integer-zero controls. Runtime behavior is unchanged.

Related to #385.

Verification on Rust 1.94.1:

• Formatting check.
• Documentation builds with RUSTDOCFLAGS='-D warnings' for default features, no default features, and libm without default features.
• Doctests in those same three configurations.
• The repository's ci/test_full.sh feature-matrix script passed before the final wording-only precision edit. Documentation and doctests were rerun on the final source.

The upstream MSRV and platform matrix was not run locally. AI-assisted implementation and verification; independent agent review.