Reformat documentation

docs/en/mods/in.md

@@ -0,0 +1,10 @@
+Modifier in
+===========
+
+The modifier is implementation of [in](../operators.md#containment-operator) operator (performs containment test).
+
+```smarty
+{if $number|in:[1, 3, 42]}
+...
+{/if}
+```