expressions: teach short-circuit evaluation of && and ||

src/expressions/comparison-and-logical.md

@@ -26,6 +26,27 @@ Operate on `bool` values:
 | `\|\|`   | `a \|\| b` |
 | `!`      | `!a`       |
 
+## Short-Circuit Evaluation
+
+`&&` and `||` short-circuit: the right operand is evaluated only when the left
+operand does not already determine the result. `a && b` skips `b` when `a` is
+`false`; `a || b` skips `b` when `a` is `true`.
+
+This makes guard expressions safe — the check on the left protects the
+operation on the right:
+
+```inference
+pub fn is_big_ratio(y: i32, x: i32) -> bool {
+    // `y / x` runs only when `x != 0` is true,
+    // so this can never divide by zero.
+    return x != 0 && y / x > 1;
+}
+```
+
+The same pattern guards array accesses: `i < len && arr[i] > 0` never reads
+out of bounds. If you need both operands evaluated unconditionally, use the
+[bitwise operators](./bitwise.md) `&` and `|` — they do not short-circuit.
+
 ## Example
 
 ```inference