add slice patterns to the guide

Fixes #19177.

Author: steveklabnik

src/doc/guide.md

@@ -3991,6 +3991,19 @@ match origin {
 }
 ```
 
+If you want to match against a slice or array, you can use `[]`:
+
+```{rust}
+fn main() {
+    let v = vec!["match_this", "1"];
+
+    match v.as_slice() {
+        ["match_this", second] => println!("The second element is {}", second),
+        _ => {},
+    }
+}
+```
+
 Whew! That's a lot of different ways to match things, and they can all be
 mixed and matched, depending on what you're doing: