Waker and LocalWaker docs: Warn against a particular, likely, abuse

In pers. comm with colleagues, it became clear that even fairly
experienced Rust programmers can be confused about the behaviour of
Futures.

No-op wakers seem like a superficially attractive answer for calling a
poll method when no waker is available.  So, at least add some docs
warning of the problems.

This may even save someone using Stable, since they may see the
warning and choose a better approach, rather than simply writing their
own no-op waker.

Author: ijackson

library/core/src/task/wake.rs

@@ -541,6 +541,16 @@ impl Waker {
     ///
     /// If an owned `Waker` is needed, `clone()` this one.
     ///
+    /// # Warning
+    ///
+    /// Do not use this function as a way to be able to call `poll` methods,
+    /// in production code,
+    /// from within a non-`async`, non-`poll`, function.
+    ///
+    /// Using a no-op waker for that purpose would cause wakeups to be lost:
+    /// the future would store only the no-op waker,
+    /// replacing the proper waker for the waiting task.
+    ///
     /// # Examples
     ///
     /// ```
@@ -798,6 +808,16 @@ impl LocalWaker {
     ///
     /// If an owned `LocalWaker` is needed, `clone()` this one.
     ///
+    /// # Warning
+    ///
+    /// Do not use this function as a way to be able to call `poll` methods,
+    /// in production code,
+    /// from within a non-`async`, non-`poll`, function.
+    ///
+    /// Using a no-op waker for that purpose would cause wakeups to be lost:
+    /// the future would store only the no-op waker,
+    /// replacing the proper waker for the waiting task.
+    ///
     /// # Examples
     ///
     /// ```