Update expect messages in library/alloc/boxed.rs and library/alloc/string.rs to follow the style guide

library/alloc/src/boxed.rs

@@ -1368,7 +1368,7 @@ impl<T: ?Sized> Box<T> {
     ///
     /// unsafe {
     ///     let non_null = NonNull::new(alloc(Layout::new::<i32>()).cast::<i32>())
-    ///         .expect("allocation failed");
+    ///         .expect("alloc should have successfully allocated memory");
     ///     // In general .write is required to avoid attempting to destruct
     ///     // the (uninitialized) previous contents of `non_null`.
     ///     non_null.write(5);

library/alloc/src/string.rs

@@ -1314,7 +1314,7 @@ impl String {
     ///
     ///     Ok(output)
     /// }
-    /// # process_data("rust").expect("why is the test harness OOMing on 4 bytes?");
+    /// # process_data("rust").expect("reserving capacity for 12 bytes should never fail");
     /// ```
     #[stable(feature = "try_reserve", since = "1.57.0")]
     pub fn try_reserve(&mut self, additional: usize) -> Result<(), TryReserveError> {
@@ -1355,7 +1355,7 @@ impl String {
     ///
     ///     Ok(output)
     /// }
-    /// # process_data("rust").expect("why is the test harness OOMing on 4 bytes?");
+    /// # process_data("rust").expect("reserving capacity for 12 bytes should never fail");
     /// ```
     #[stable(feature = "try_reserve", since = "1.57.0")]
     pub fn try_reserve_exact(&mut self, additional: usize) -> Result<(), TryReserveError> {