Replace Cluster XML documentation placeholders

[Aaronontheweb]

Akka.Cluster contained 481 XML documentation placeholders in 258 blocks across 16 files, including one Cluster.Metrics return description. Replace them with source-checked descriptions of membership, reachability and vector clocks, heartbeats and failure detectors, cluster lifecycle, routing placement, and settings. Preserve existing useful prose and leave runtime source, signatures, attributes, nullable directives, configuration, tests, and API approval baselines unchanged.

Luna authored the docs. Independent Sol review covered every selected block and rechecked all seven correction groups, including removal margins, heartbeat initialization, up-number retention, and routing behavior. This is an independent package batch targeting dev.

Validation:

• Roslyn comparison: all 16 changed files preserve executable tokens, ordinary comments, and directives. Exact assigned-file scan: zero XML documentation placeholders.
• Both Cluster and Cluster.Metrics Release builds with warnings as errors: zero warnings/errors, at baseline and candidate.
• Compiler XML: zero placeholders in both packages and unchanged member IDs.
• Scoped DocFX metadata and rendered build with warnings as errors: zero warnings/errors, at baseline and candidate.
• Existing Cluster tests: 436 passed in the ongoing campaign incremental run at Core documentation head 88466ae39. Both that head and this candidate preserve all runtime source relative to the same d70c03ed base; the test result is carried across those documentation variants. The wider campaign run is complete: all 34 selected projects have passing results, 8,023 passed and 523 existing skips. Serialization V2 required correcting the test invocation to avoid propagating net10.0 to its netstandard2.0 generator; its 566 tests then passed. CI also runs this branch's affected tests.
• Slopwatch: zero introduced findings; the existing ClusterDaemon catch-and-log warning is identical to baseline. No suppressions or baseline edits.

The agent will not attempt to merge campaign PRs.