Render experimental RPC methods as a styled rustdoc admonition

Codegen previously emitted a single bare `/// Stability: experimental.`
line on each experimental wire method. stephentoub flagged this as
weak signal -- there's no way for a consumer to tell from the rustdoc
that they're using something that may change.

Switch the rustdoc emit to a `<div class="warning">...</div>` block,
which renders on docs.rs as a yellow-bordered callout (the standard
rustdoc admonition convention). Matches the visibility Python's
generated docs already get via Sphinx's `.. warning::` directive.

This is documentation-quality only; no compile-time opt-in. Stable
Rust has no first-class `#[experimental]` attribute (`#[unstable]`
is rustc-internal, std-only). If we ever want compile-time friction
the ecosystem patterns are a Cargo feature gate (e.g. `tokio_unstable`-
style `features = ["experimental"]`) or an `unstable` module path;
both are larger codegen changes affecting every experimental RPC
across all SDK surfaces. Punted for 0.1.0.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: tclem