docs: complete Akka.Persistence XML documentation placeholders

[Aaronontheweb]

Akka.Persistence still had 545 TBD placeholders in 288 XML documentation blocks across 25 files. Replace them with source-checked descriptions of snapshot/plugin hooks, persistent actor callbacks, journals and event adapters, delivery, and persistent FSM state transitions. Preserve useful existing prose and leave runtime code, public signatures, nullable directives/annotations, configuration, tests, and API approval baselines unchanged.

Luna authored the documentation; independent Sol reviews inspected every selected block and rechecked the corrections. This is the first of 13 independent package/area batches targeting dev.

Validation:

• Roslyn comparison: all 25 changed source files have identical executable tokens, ordinary comments, attributes, and directives; six positive/negative controls pass.
• Release build with warnings as errors: zero warnings/errors.
• API approvals: 33 passed. Existing Persistence tests: 306 passed, three existing skips.
• Incrementalist from an isolated ordinary clone, explicitly selecting Akka.slnx: 14 affected test projects, 1,492 passed, eight existing skips, zero failures. Worktree mode was avoided because the pinned tool reads the primary checkout's Git state.
• Compiler XML: zero placeholders, all 771 member IDs unchanged. A Roslyn consumer resolves the new snapshot argument and protected plugin-hook docs from the Release DLL/XML. Actual IDE tooltip rendering was not exercised.
• Scoped Persistence DocFX metadata/rendered build with warnings as errors: zero warnings/errors. Full rendered documentation build also passes; full metadata still fails on the identical 576 pre-existing warnings, with zero introduced warnings.
• Slopwatch: zero introduced findings; the existing SW003 catch-and-log warning in PersistencePluginProxy.cs:168 is unchanged. No suppressions or baseline edits.

The agent will not attempt to merge campaign PRs.

Current PR head: all required CI checks pass, including Linux/Windows unit tests, all four multi-node jobs, DocFX, packaging, AOT, CodeQL, documentation checks, and Slopwatch.