ENH: Fill the 58 missing public API docstrings

[mmcky]

Problem

Public docstring coverage is 239 of 297 public non-dunder definitions (80.5%). The 58 gaps concentrate in two modules:

Module	Missing
_graph_tools.py	17 (mostly properties)
markov/core.py	16
_arma.py, _kalman.py, distributions.py, util/timing.py	4 each
markov/ddp.py, random/utilities.py, others	3 each

Where docstrings exist they are numpydoc-conformant and generally good; this is about completeness, not quality.

Good for outside contributors, especially anyone with domain knowledge of the graph and Markov chain code, since several gaps are on properties whose meaning is not obvious from the name. Worth splitting into per-module sub-issues so two people do not collide.

Acceptance criteria

• All 58 public definitions carry a numpydoc-conformant docstring
• Parameters, returns and raised exceptions documented where non-obvious; examples for the public entry points among them
• The Sphinx build produces no new warnings
• A docstring-coverage check considered for CI to prevent regression

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From the July 2026 technical-debt audit (AI-assisted; claims verified against 28d4b3b on 2026-07-25).