langref: add documentation for unions with inferred tag and explicit tag values

doc/langref/test_tagged_union_with_tag_values.zig

@@ -0,0 +1,17 @@
+const std = @import("std");
+const expect = std.testing.expect;
+
+const Tagged = union(enum(u32)) {
+    int: i64 = 123,
+    boolean: bool = 67,
+};
+
+test "tag values" {
+    const int: Tagged = .{ .int = -40 };
+    try expect(@intFromEnum(int) == 123);
+
+    const boolean: Tagged = .{ .boolean = false };
+    try expect(@intFromEnum(boolean) == 67);
+}
+
+// test