Rollup merge of #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