wasm: document that Tokio may use blocking ops

tokio/src/lib.rs

@@ -432,6 +432,11 @@
 //! Enabling any other feature (including `full`) will cause a compilation
 //! failure.
 //!
+//! When using `WASM` with multiple threads, support for blocking calls is
+//! required. This means that, for example, using a Tokio mpsc channel to send
+//! messages from a web worker to the main browser context will not work, as the
+//! main browser context does not support blocking.
+//!
 //! The `time` module will only work on `WASM` platforms that have support for
 //! timers (e.g. wasm32-wasi). The timing functions will panic if used on a `WASM`
 //! platform that does not support timers.

tokio/src/sync/mod.rs

@@ -445,6 +445,12 @@
 //! As an exception, methods ending in `_timeout` are not runtime agnostic
 //! because they require access to the Tokio timer. See the documentation of
 //! each `*_timeout` method for more information on its use.
+//!
+//! This module is also compatible with WASM, with one caveat: Tokio may use
+//! blocking operations when more than one thread is involved. This means that,
+//! for example, using a Tokio mpsc channel to send messages from a web worker
+//! to the main browser context will not work, as the main browser context does
+//! not support blocking operations.
 
 cfg_sync! {
     /// Named future types.