Changes

Add missing `item-attribute` rule for `let-binding` in documentation of attributes (#14077)

This reflects the grammar in
https://github.com/ocaml/ocaml/blob/8761443617f229d5fe683ed2570aa79c8d64348a/parsing/parser.mly#L2742-L2759
and without this rule, the documentation doesn't account for forms like

```
let foo = bar
[@@bax]
```
 (commit: 48efc58)