doc: document type Fn = fn (string) string (type aliases) (#10471)

examples/function_types.v

@@ -0,0 +1,32 @@
+// Function signatures can be declared as types:
+
+type Filter = fn (string) string
+
+// Functions can accept function types as arguments:
+
+fn filter(s string, f Filter) string {
+	return f(s)
+}
+
+// Declare a function with a matching signature:
+
+fn uppercase(s string) string {
+	return s.to_upper()
+}
+
+fn main() {
+	// A function can be assigned to a matching type:
+
+	my_filter := Filter(uppercase)
+
+	// You don't strictly need the `Filter` cast - it's only used
+	// here to illustrate how these types are compatible.
+
+	// All of the following prints "HELLO WORLD":
+
+	println(filter('Hello world', my_filter))
+	println(filter('Hello world', uppercase))
+	println(filter('Hello world', fn (s string) string {
+		return s.to_upper()
+	}))
+}