Rollup merge of #104708 - jonasspinner:fix-backoff-doc-to-match-implementation, r=compiler-errors

Fix backoff doc to match implementation

The commit 8dddb22943 in the crossbeam-channel PR (#93563) changed the backoff strategy to be quadratic instead of exponential. This updates the doc to prevent confusion.

library/std/src/sync/mpmc/utils.rs

@@ -136,7 +136,7 @@ impl Backoff {
         }
     }
 
-    /// Returns `true` if exponential backoff has completed and blocking the thread is advised.
+    /// Returns `true` if quadratic backoff has completed and blocking the thread is advised.
     #[inline]
     pub fn is_completed(&self) -> bool {
         self.step.get() > YIELD_LIMIT