Rollup merge of #131462 - cuviper:open_buffered-error, r=RalfJung

Mention allocation errors for `open_buffered`

This documents that `File::open_buffered` may return an error on allocation failure.

library/std/src/fs.rs

@@ -394,7 +394,8 @@ pub fn open<P: AsRef<Path>>(path: P) -> io::Result<File> {
     ///
     /// # Errors
     ///
-    /// This function will return an error if `path` does not already exist.
+    /// This function will return an error if `path` does not already exist,
+    /// or if memory allocation fails for the new buffer.
     /// Other errors may also be returned according to [`OpenOptions::open`].
     ///
     /// # Examples