docs: document all keyword

docs/src/guides/code-highlight.md

@@ -90,9 +90,10 @@ Similar to the syntax used for selective highlighting, dynamic highlighting will
 code block are highlighted every time you move to the next/previous slide.
 
 This is achieved by using the separator `|` to indicate what sections of the code will be highlighted at a given time.
+You can also use `all` to highlight all lines.
 
 ~~~markdown
-```rust {1,3|5-7}
+```rust {1,3|5-7|all}
    fn potato() -> u32 {
 
        println!("Hello world");