docs: add note about current-thread + Handle::block_on
There's already an existing warning about this combo in the
documentation for `Handle::block_on`. This commit adds a summarized
version in `Runtime::handle`.
@@ -138,6 +138,9 @@ impl Runtime {
/// The returned handle can be used to spawn tasks that run on this runtime, and can
/// be cloned to allow moving the `Handle` to other threads.
///
+ /// Calling [`Handle::block_on`] on a handle to a `current_thread` runtime is error-prone.
+ /// Refer to the documentation of [`Handle::block_on`] for more.
+ ///
/// # Examples
///
/// ```
