runtime: update the documentation around Handle (#2328)

This PR was prompted by having encountered a few cases of people not noticing that Runtime::handle can be cloned, and therefore not realizing it could be moved to another thread.

1 files changed, 4 insertions, 0 deletions

diff --git a/tokio/src/task/spawn.rs b/tokio/src/task/spawn.rs
index 4e19f559..fa5ff13b 100644
--- a/tokio/src/task/spawn.rs
+++ b/tokio/src/task/spawn.rs
@@ -16,6 +16,10 @@ doc_rt_core! {
     /// When a runtime is shutdown, all outstanding tasks are dropped,
     /// regardless of the lifecycle of that task.
     ///
+    /// This function must be called from the context of a Tokio runtime. Tasks running on
+    /// the Tokio runtime are always inside its context, but you can also enter the context
+    /// using the [`Handle::enter`](crate::runtime::Handle::enter()) method.
+    ///
     /// # Examples
     ///
     /// In this example, a server is started and `spawn` is used to start a new task