tokio: document missing timer panics (#2801)

Fixes: #2696
Co-authored-by: Mikail Bagishov <[email protected]>

tokio/src/time/driver/handle.rs

@@ -20,6 +20,17 @@ impl Handle {
     /// # Panics
     ///
     /// This function panics if there is no current timer set.
+    ///
+    /// It can be triggered when `Builder::enable_time()` or
+    /// `Builder::enable_all()` are not included in the builder.
+    ///
+    /// It can also panic whenever a timer is created outside of a Tokio
+    /// runtime. That is why `rt.block_on(delay_for(...))` will panic,
+    /// since the function is executed outside of the runtime.
+    /// Whereas `rt.block_on(async {delay_for(...).await})` doesn't
+    /// panic. And this is because wrapping the function on an async makes it
+    /// lazy, and so gets executed inside the runtime successfuly without
+    /// panicking.
     pub(crate) fn current() -> Self {
         context::time_handle()
             .expect("there is no timer running, must be called from the context of Tokio runtime")