Rollup merge of rust-lang#122512 - baitcode:2024-03-14-buffer-documentation-fix, r=Nilstrieb

Cursor.rs documentation fix

Reason:

I've been learning Rust std library and got confused. Seek trait documentation clearly states that negative indexes will cause an error. And the code in the Cursor example uses negative index. I found myself trying to understand what am I missing until I've actually executed the code and got error. I decided to submit small fix to the documentation.

Author: GuillaumeGomez

library/std/src/io/cursor.rs

@@ -51,6 +51,8 @@ use crate::io::{self, BorrowedCursor, ErrorKind, IoSlice, IoSliceMut, SeekFrom};
 /// // We might want to use a BufReader here for efficiency, but let's
 /// // keep this example focused.
 /// let mut file = File::create("foo.txt")?;
+/// // First, we need to allocate 10 bytes to be able to write into.
+/// file.set_len(10)?;
 ///
 /// write_ten_bytes_at_end(&mut file)?;
 /// # Ok(())