Update snippet descriptions

snippets/union.md

@@ -1,11 +1,11 @@
 ---
 title: union
-tags: array,math,beginner
+tags: array,beginner
 ---
 
-Returns every element that exists in any of the two arrays once.
+Returns every element that exists in any of the two arrays at least once.
 
-- Create a `Set` with all values of `a` and `b` and convert to an array.
+- Create a `new Set()` with all values of `a` and `b` and convert it to an array.
 
 ```js
 const union = (a, b) => Array.from(new Set([...a, ...b]));