eframe: Fix building the wasm32 docs for docs.rs (#3757)

* Closes #3756

Fix suggested by @morionelab

crates/eframe/Cargo.toml

@@ -21,6 +21,7 @@ include = [
 
 [package.metadata.docs.rs]
 all-features = true
+rustc-args = ["--cfg=web_sys_unstable_apis"]
 targets = ["x86_64-unknown-linux-gnu", "wasm32-unknown-unknown"]
 
 [lib]

crates/eframe/src/lib.rs

@@ -9,6 +9,21 @@
 //! In short, you implement [`App`] (especially [`App::update`]) and then
 //! call [`crate::run_native`] from your `main.rs`, and/or use `eframe::WebRunner` from your `lib.rs`.
 //!
+//! ## Compiling for web
+//! To get copy-paste working on web, you need to compile with
+//! `export RUSTFLAGS=--cfg=web_sys_unstable_apis`.
+//!
+//! You need to install the `wasm32` target with `rustup target add wasm32-unknown-unknown`.
+//!
+//! Build the `.wasm` using `cargo build --target wasm32-unknown-unknown`
+//! and then use [`wasm-bindgen`](https://github.com/rustwasm/wasm-bindgen) to generate the JavaScript glue code.
+//!
+//! See the [`eframe_template` repository](https://github.com/emilk/eframe_template/) for more.
+//!
+//! ## Simplified usage
+//! If your app is only for native, and you don't need advanced features like state persistence,
+//! then you can use the simpler function [`run_simple_native`].
+//!
 //! ## Usage, native:
 //! ``` no_run
 //! use eframe::egui;
@@ -114,10 +129,6 @@
 //! }
 //! ```
 //!
-//! ## Simplified usage
-//! If your app is only for native, and you don't need advanced features like state persistence,
-//! then you can use the simpler function [`run_simple_native`].
-//!
 //! ## Feature flags
 #![cfg_attr(feature = "document-features", doc = document_features::document_features!())]
 //!