Final-review cleanup on refactor/remove-drf-writable-nested: several
comments/docstrings still described drf-writable-nested's removed
NestedUpdateMixin behavior in the present tense. Rewrote them to explain
the shared context-cache and delete-query-count guarantees in terms of
the current code (bulk-edit's per-field CustomFieldInstanceSerializer
construction, and _sync_custom_fields' single hard-delete query).
Also collapsed the duplicated `if custom_fields_data is not None:
self._sync_custom_fields(...)` line in DocumentSerializer.update()'s
audit-log branches into a single code path using
contextlib.nullcontext(), removing the drift risk that caused the
earlier audit-actor bug.
No behavior change.
stumpylog
74abdf8ff8