Trim the identity work's comments to the contributing guidelines

CONTRIBUTING.md asks for one or two sentences over an essay, a pointer
rather than a re-derivation, and module docstrings that orient rather than
document the design. The identity change was written before that guidance
was read, and its comments narrate the reasoning at roughly twice the
length the conclusions need.

Cut to the conclusion and the evidence that makes it credible, keeping
every issue reference: _resolve_identity's docstring 41 lines -> 22 (now
shorter than the two longest already in that file), rekey_entry 23 -> 14,
its module docstring 21 -> 10, resolve_device_key 24 -> 16,
is_usable_device_id 15 -> 8, and the same treatment for the inline blocks
in _resolve_identity, the CONF_DEVICE_KEY note, the redaction rationale,
and the migration suite's module docstring.

Comments only; the suite and the thirteen-mutation sweep are unchanged.

tests/localthings/test_identity_migration.py

@@ -1,24 +1,16 @@
 """Moving an existing install onto the OCF device UUID (issue #381).
 
-Every other migration this integration has done finishes inside
-`async_migrate_entry`. This one can't: the UUID is only readable from the
-appliance, and an entry can load entirely from its stored snapshot while
-the appliance is off (issue #295). So the config-entry version bumps up
-front and the coordinator adopts the UUID on the first *live* poll,
-rewriting the entity registry, the device registry and the entry's own
+This migration can't finish inside `async_migrate_entry` -- the UUID is
+only readable from the appliance, and an entry can load from its snapshot
+while that appliance is off (issue #295) -- so the coordinator adopts it
+on the first live poll, rewriting both registries and the entry's
 unique_id together.
 
-That makes this the riskiest migration in the codebase: it rewrites the
-identity of registry rows a user's automations, dashboards, history and
-areas all hang off. The suite is therefore organised around what an
-existing user must not lose, rather than around the functions involved:
-
-* the entity_id, and every customization carried on that registry row
-* long-term statistics (keyed by entity_id -- see the end-to-end test in
-  tests/test_rekey_statistics_end_to_end.py, which drives a real recorder)
-* the device row, its area, and a composite appliance's subdevice links
-* other config entries' rows, which this must never touch
-* stability: an upgrade that happens twice, or offline, must not churn
+That makes it the riskiest migration here: it rewrites the identity of
+rows a user's automations, history and areas hang off. These tests are
+organised around what must not break rather than around the functions
+involved. Statistics are covered separately, against a real recorder, in
+tests/test_rekey_statistics_end_to_end.py.
 """
 
 from __future__ import annotations