Document unstable APIs in master docs (#2025)

Author: alexcrichton

azure-pipelines.yml

@@ -281,6 +281,8 @@ jobs:
         displayName: "Document js-sys"
       - script: cargo doc --no-deps --manifest-path crates/web-sys/Cargo.toml --all-features
         displayName: "Document web-sys"
+        env:
+          RUSTDOCFLAGS: --cfg=web_sys_unstable_apis
       - script: cargo doc --no-deps --manifest-path crates/futures/Cargo.toml
         displayName: "Document wasm-bindgen-futures"
       # Make a tarball even though a zip is uploaded, it looks like the tarball