Explain interaction with if and | in patterns

Fixes #26012

Author: steveklabnik

src/doc/trpl/patterns.md

@@ -154,6 +154,31 @@ match x {
 
 This prints `Got an int!`.
 
+If you’re using `if` with multiple patterns, the `if` applies to both sides:
+
+```rust
+let x = 4;
+let y = false;
+
+match x {
+    4 | 5 if y => println!("yes"),
+    _ => println!("no"),
+}
+```
+
+This prints `no`, because the `if` applies to the whole of `4 | 5`, and not to
+just the `5`, In other words, the the precedence of `if` behaves like this:
+
+```text
+(4 | 5) if y => ...
+```
+
+not this:
+
+```text
+4 | (5 if y) => ...
+```
+
 # ref and ref mut
 
 If you want to get a [reference][ref], use the `ref` keyword: