docs(stack/gate): note Tx::lost() cancel safety (#4482)

cancel safety is a relevant quality of `Future`s returned by `async`
functions. this affects whether a future is safe to use within a
`tokio::select!` block.

this function is used in some of these macro invocations, like the
`ConsecutiveFailures` circuit-breaker.

this function is cancel safe because it is strictly delegating down to
`tokio::sync::watch::Sender::closed()`, which is itself cancel safe:
<https://docs.rs/tokio/latest/tokio/sync/watch/struct.Sender.html#method.closed>.

Signed-off-by: katelyn martin <kate@buoyant.io>

Author: cratelyn

linkerd/stack/src/gate.rs

@@ -120,6 +120,10 @@ impl Rx {
 
 impl Tx {
     /// Returns when all associated `Rx` clones are dropped.
+    ///
+    /// # Cancel safety
+    ///
+    /// This method is cancel safe.
     pub async fn lost(&self) {
         self.0.closed().await
     }