docs: fix signum documentation to say zero returns 0

[codeAnqiang-ma]

Which issue does this PR close?

N/A — documentation-only correction; no open issue tracks it.

Rationale for this change

The user guide for signum says "Zero and positive numbers return 1", but
signum(0) returns 0 — the documented return value for zero is the opposite
of what the function does. The repository's own tests already pin the real
behavior:

• datafusion/sqllogictest/test_files/scalar.slt:
  select signum(-2), signum(0), signum(2); → -1 0 1
• The unit tests in signum.rs assert signum(±0.0) == 0.0

That behavior is intentional: it matches PostgreSQL sign() and Spark
signum(), and was changed on purpose in #11580 (issue #11557). Only the docs
were left describing the pre-#11580 behavior, so this PR updates the docs, not
the code.

What changes are included in this PR?

Documentation only:

• Update the #[user_doc] description in datafusion/functions/src/math/signum.rs
  to state that zero returns 0.
• Regenerate docs/source/user-guide/sql/scalar_functions.md via
  dev/update_function_docs.sh.

Are these changes tested?

No new tests: the corrected wording describes behavior the existing tests above
already assert, and those are what it was checked against.

• cargo test -p datafusion-functions --lib math::signum → 2 passed
• cargo fmt --all -- --check and ./ci/scripts/doc_prettier_check.sh → clean

Are there any user-facing changes?

Yes, documentation only. No API or runtime behavior change.

---

Prepared with AI assistance; every claim above was verified locally against the
repository's existing tests before opening this PR.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.14%. Comparing base (ab12f5e) to head (90d4e13).
⚠️ Report is 18 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #24324      +/-   ##
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- Coverage   81.14%   81.14%   -0.01%     
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  Files        1112     1112              
  Lines      386933   386933              
  Branches   386933   386933              
==========================================
- Hits       313967   313958       -9     
- Misses      54476    54481       +5     
- Partials    18490    18494       +4     

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[Jefffrey]

word of advice if there is a human looking at this; the PR description doesnt need to be that verbose for such a simple change. theres elegance in being succinct