doc: mark out experimental diagnostics to avoid confusion

rust-analyzer version: 0.3.2137-standalone (2b750da1a 2024-10-06) [/Users/chenyuwang/.vscode/extensions/rust-lang.rust-analyzer-0.3.2137-darwin-arm64/server/rust-analyzer]

rustc version: -

editor or extension: -

relevant settings: -
repository link (if public, optional): -
code snippet to reproduce:

fn main() {
    let n = 3;
    let b = n();
   // show `expected function, found i32rust-analyzer[E0618](https://doc.rust-lang.org/stable/error_codes/E0618.html)`
   // only when "rust-analyzer.diagnostics.experimental.enable": true
}

I'd like to test out the diagnostics https://rust-analyzer.github.io/manual.html#expected-function but it seems it is actually "experimental" and is only enabled when "rust-analyzer.diagnostics.experimental.enable": true is set to true in .vscode/settings.json.
I think the documentation is supposed to make it clear with the configuration of experimental diagnostics in the auto-generated documentation. If possible I think I can make a PR to help.

Start with the rust-analyzer manual's expected-function section and the documentation for rust-analyzer.diagnostics.experimental.enable. Update the auto-generated documentation so experimental diagnostics are identified clearly, then verify that the setting and diagnostic behavior are described consistently.