docs(axum): recommend actor pattern for concurrent WebSocket read/write - #3842

Motivation

The WebSocket module docs suggest StreamExt::split as the way to read and write concurrently. Splitting wraps each half in a lock that must be acquired on every operation, which adds avoidable overhead. Upstream issue #3793 proposes highlighting the actor pattern as the higher-performance alternative, and #3786 already laid the groundwork by documenting that WebSocket::recv is cancel safe.

Solution

Rework the "Read and write concurrently" section in axum/src/extract/ws.rs:

• Lead with the actor pattern: a single task owns the socket and uses tokio::select! to race WebSocket::recv (cancel safe) against an mpsc channel that other tasks use to send messages to the client. Links to Actors with Tokio for background.
• Keep StreamExt::split as a documented alternative, with a note about its per-operation locking overhead.

Both examples are compile-checked doc tests. Verified with cargo test -p axum --all-features --doc ws and cargo doc using CI's rustdoc flags (-D rustdoc::all -A rustdoc::private-doc-tests); the test-only hidden line does not appear in the rendered docs.