docs: full Doxygen API documentation for reflection data

[CV-GPhL]

Summary

This PR adds complete Doxygen API documentation for Gemmi's reflection data layer, covering 9 headers:

• mtz.hpp — MTZ binary reflection file format (Mtz, Column, Dataset, Batch, MtzDataProxy)
• refln.hpp — CIF/mmCIF reflection block abstraction (ReflnBlock, ReflnDataProxy)
• cif2mtz.hpp — CIF-to-MTZ conversion (CifToMtz, column mapping, old-style anomalous handling)
• mtz2cif.hpp — MTZ-to-CIF/SF-mmCIF conversion (MtzToCif with configurable spec)
• xds_ascii.hpp — XDS_ASCII.HKL file reading (XdsAscii, Refl, orientation matrices)
• xds2mtz.hpp — XDS-to-MTZ conversion (xds_to_mtz with batch header support)
• intensit.hpp — Intensity merging and scaling (Intensities, MergingStats, AnisoScaling)
• binner.hpp — Resolution shell binning (Binner, Method enum, setup/query API)
• asudata.hpp — Asymmetric unit data storage (AsuData, HklValue, load/ensure_asu)

Documentation standard

• /// triple-slash Doxygen comments throughout
• Every public entity has @brief; non-obvious parameters have @param/@return/@throws/@tparam
• Semantics verified against implementations (MTZ column type codes, resolution as 1/d², batch header offsets, CIF null → NaN, ASU phase shifts)

Dependencies

This PR depends on PR #413 (Doxygen/Breathe infrastructure) being merged first.

It follows PR #414 (core data structures), PR #415 (CIF handling), and PR #416 (structure I/O) in the documentation series.

Series context

This is PR 5 of a planned series of 10 PRs adding incremental API documentation:

PR	Branch	Content
#413	api-docs/infra	Build infrastructure (Doxyfile, conf.py, api.rst, readthedocs)
#414	api-docs/core-structures	model.hpp, unitcell.hpp, symmetry.hpp, metadata.hpp, elem.hpp, seqid.hpp, resinfo.hpp, small.hpp
#415	api-docs/cif	cif.hpp, cifdoc.hpp, read_cif.hpp, to_cif.hpp, to_json.hpp, json.hpp, numb.hpp, ddl.hpp
#416	api-docs/structure-io	mmcif.hpp, mmread.hpp, mmread_gz.hpp, pdb.hpp, to_mmcif.hpp, to_pdb.hpp, crd.hpp, smcif.hpp, mmdb.hpp, pirfasta.hpp
This PR	api-docs/reflection	mtz.hpp, refln.hpp, cif2mtz.hpp, mtz2cif.hpp, xds_ascii.hpp, xds2mtz.hpp, intensit.hpp, binner.hpp, asudata.hpp
PR 6	api-docs/maps-grids	grid.hpp, ccp4.hpp, dencalc.hpp, fourier.hpp, and related
PR 7	api-docs/calculations	calculate.hpp, neighbor.hpp, sfcalc.hpp, align.hpp, and related
PR 8	api-docs/chemistry	chemcomp.hpp, monlib.hpp, topo.hpp, and related
PR 9	api-docs/scattering-math	formfact.hpp, math.hpp, cellred.hpp, and related
PR 10	api-docs/io-utils	fileutil.hpp, gz.hpp, util.hpp, logger.hpp, and related

Attribution

Builds on and credits prior work in PR #402 (Paul Emsley / pemsley). All comments audited/rewritten where needed against actual implementations.