Update the documentation to reflect web support (#1183)

* Update the documentation to reflect web support

Indicate which methods have platform-specific web behavior

* cargo fmt

src/platform/mod.rs

@@ -7,6 +7,7 @@
 //!  - `macos`
 //!  - `unix`
 //!  - `windows`
+//!  - `web`
 //!
 //! And the following platform-specific module:
 //!

src/platform/web.rs

@@ -1,5 +1,10 @@
 #![cfg(target_arch = "wasm32")]
 
+//! The web target does not automatically insert the canvas element object into the web page, to
+//! allow end users to determine how the page should be laid out. Use the `WindowExtStdweb` or
+//! `WindowExtWebSys` traits (depending on your web backend) to retrieve the canvas from the
+//! Window.
+
 #[cfg(feature = "stdweb")]
 use stdweb::web::html_element::CanvasElement;