Deep flatten and housekeeping

Changed original flatten to be named deepFlatten, added normal flatten, improved some other snippets.

snippets/flatten-array.md

@@ -1,10 +1,8 @@
 ### Flatten array
 
-Use recursion.
-Use `reduce()` to get all elements that are not arrays, flatten each element that is an array.
+Use `reduce()` to get all elements inside the array and `concat()` to flatten them.
 
 ```js
-const flatten = arr =>
-  arr.reduce( (a, v) => a.concat( Array.isArray(v) ? flatten(v) : v ), []);
-// flatten([1,[2],[[3],4],5]) -> [1,2,3,4,5]
+const flatten = arr => arr.reduce( (a, v) => a.concat(v), []);
+// flatten([1,[2],3,4) -> [1,2,3,4]
 ```