Document channel sink behavior and distributed data reads

[Aaronontheweb]

Document ChannelSink.AsReader buffering, Wait backpressure, drop modes, the single-reader promise, and the difference between channel closure and reader completion.

Add read-consistency XML contracts for local, fixed-count, majority, majority-plus, and all-replica reads, including MinCapacity and Additional calculation order. Clarify DistributedData termination and local key enumeration.

Refs #647
Refs #4288

Validation of the combined documentation candidate:

• Release builds of Akka, Akka.Persistence, Akka.Streams, and Akka.DistributedData pass with warnings treated as errors; all 33 API approval tests pass.
• Roslyn comparisons confirm identical C# tokens and non-XML trivia; six verifier controls pass. No runtime, public API, signature, or nullability changes.
• Two independent GPT-6.1 Sol reviewers checked this batch against implementation; all identified corrections were addressed and rechecked.
• Markdown lint, Slopwatch, and the staged DocFX website build pass. Existing article anchors and maintained code samples are preserved.
• Fresh API metadata for the four affected projects introduces no reference warnings. Metadata generation with warnings as errors still fails on the same pre-existing ActorSystem.cs reference to ConfigurationFactory.Load() as the baseline. Other API metadata was cached for cross-reference resolution during the website render.

[Aaronontheweb]

LGTM