Add Args/Returns sections to calibration API docstrings

[xuanku-001]

Summary

Adds Args/Returns/Raises sections to the docstrings of the public
calibration API, following the style already used elsewhere in the codebase
(e.g. components/dld.py, components/pulses.py, components/xgm.py).

• calibration/calcat.py: setup_client, set_default_caldb_root, and the
  five CalCatAPIClient methods (format_time, request, get, get_paged,
  post)
• calibration/constants.py: SingleConstant.ndarray,
  CalibrationData.require_calibrations, CalibrationData.select_calibrations
• calibration/detector.py: DetectorData.list_by_instrument

All existing sentences are preserved verbatim; only sections are added, so no
behaviour or signatures change. These docstrings are rendered on the
documentation site via mkdocstrings, where parameters, return values and
raised errors are currently not described.

I deliberately left the select_trains/split_trains methods and the
constant metadata helpers untouched: their docstrings refer to the upstream
extra_data documentation on purpose, and adding parameter sections there
would just duplicate it.

Testing

• python -m py_compile passes on all touched files
• AST statement counts are unchanged (documentation only, no executable lines
  added)

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This contribution was prepared with AI assistance (a local LLM-based agent harness with deterministic checks and human review).