docs: document Awkward how=zip grouping

[sapunyangkut]

Summary

• document the supported how="zip" value in every existing how parameter description
• clarify that Awkward uses ak.zip to interleave compatible branches
• keep runtime behavior unchanged

Validation

• python -m compileall -q src/uproot/behaviors/RNTuple.py src/uproot/behaviors/TBranch.py src/uproot/interpretation/library.py
• focused AST check: all 10 how docstrings contain the new guidance and the existing how == "zip" implementation still calls awkward.zip
• git diff --check

The full Sphinx build was not run locally because its optional documentation dependencies are not installed in this checkout; the added text uses only existing inline-literal markup.

Closes #1136

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This pull request was prepared with AI assistance. The issue scope, current implementation, all open pull requests, repository policies, diff, and validation results were reviewed before submission.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.82%. Comparing base (ecf2fd5) to head (27c6ad7).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

Files with missing lines	Coverage Δ
src/uproot/behaviors/RNTuple.py	72.85% <ø> (ø)
src/uproot/behaviors/TBranch.py	83.50% <ø> (ø)
src/uproot/interpretation/library.py	63.96% <ø> (ø)

... and 1 file with indirect coverage changes