Auto merge of #123851 - NobodyXu:patch-1, r=BurntSushi

Update document for std::io::Error::downcast
Resolve concern raised by `@BurntSushi` #99262 (comment)

Author: bors

library/std/src/io/error.rs

@@ -852,15 +852,19 @@ impl Error {
 }
 }
 
-/// Attempt to downcast the inner error to `E` if any.
+/// Attempt to downcast the custom boxed error to `E`.
 ///
-/// If this [`Error`] was constructed via [`new`] then this function will
-/// attempt to perform downgrade on it, otherwise it will return [`Err`].
+/// If this [`Error`] contains a custom boxed error,
+/// then it would attempt downcasting on the boxed error,
+/// otherwise it will return [`Err`].
 ///
-/// If the downcast succeeds, it will return [`Ok`], otherwise it will also
-/// return [`Err`].
+/// If the custom boxed error has the same type as `E`, it will return [`Ok`],
+/// otherwise it will also return [`Err`].
+///
+/// This method is meant to be a convenience routine for calling
+/// `Box<dyn Error + Sync + Send>::downcast` on the custom boxed error, returned by
+/// [`Error::into_inner`].
 ///
-/// [`new`]: Error::new
 ///
 /// # Examples
 ///