Rollup merge of rust-lang#53507 - phungleson:fix-impl-from-for-waker, r=cramertj

Add doc for impl From for Waker

As part of issue rust-lang#51430 (cc @skade).

The impl is very simple, so not sure if we need to go into any details.

Author: pietroalbini

src/libcore/task/wake.rs

@@ -188,6 +188,11 @@ impl LocalWaker {
 }
 
 impl From<LocalWaker> for Waker {
+    /// Converts a `LocalWaker` into a `Waker`.
+    ///
+    /// This conversion turns a `!Sync` `LocalWaker` into a `Sync` `Waker`, allowing a wakeup
+    /// object to be sent to another thread, but giving up its ability to do specialized
+    /// thread-local wakeup behavior.
     #[inline]
     fn from(local_waker: LocalWaker) -> Self {
         local_waker.0