Rollup merge of #77660 - nilslice:patch-1, r=jyn514

(docs): make mutex error comment consistent with codebase

Although exceptionally minor, I found this stands out from other error reporting language used in doc comments. With the existence of the `failure` crate, I suppose this could be slightly ambiguous. In any case, this change brings the particular comment into a consistent state with other mentions of returning errors.

library/std/src/sync/mutex.rs

@@ -276,7 +276,7 @@ pub fn lock(&self) -> LockResult<MutexGuard<'_, T>> {
     /// # Errors
     ///
     /// If another user of this mutex panicked while holding the mutex, then
-    /// this call will return failure if the mutex would otherwise be
+    /// this call will return an error if the mutex would otherwise be
     /// acquired.
     ///
     /// # Examples