docs: Add information on pure-callables to documentation (see #9132)

docs/annotating_code/type_syntax/callable_types.md

@@ -25,3 +25,34 @@ function delayedAdd(int $x, int $y) : Closure {
 $adder = delayedAdd(3, 4);
 echo $adder(true);
 ```
+
+## Pure callables
+
+For situations where the `callable` needs to be pure or immutable, the subtypes `pure-callable` and `pure-Closure` are also available.
+
+This can be useful when the `callable` is used in a function marked with `@psalm-pure` or `@psalm-mutation-free`, for example:
+
+```php
+<?php
+/** @psalm-immutable */
+class intList {
+    /** @param list<int> $items */
+    public function __construct(private array $items) {}
+    
+    /**
+     * @param pure-callable(int, int): int $callback
+     * @psalm-mutation-free
+     */
+    public function walk(callable $callback): int {
+        return array_reduce($this->items, $callback, 0);
+    }
+}
+
+$list = new intList([1,2,3]);
+
+// This is ok, as the callable is pure
+echo $list->walk(fn (int $c, int $v): int => $c + $v);
+
+// This will cause an InvalidArgument error, as the closure calls an impure function
+echo $list->walk(fn (int $c, int $v): int => $c + random_int(1, $v));
+```