Update allEqual.md

Resolve #964

snippets/allEqual.md

@@ -3,6 +3,7 @@
 Check if all elements in an array are equal.
 
 Use `Array.prototype.every()` to check if all the elements of the array are the same as the first one.
+Elements in the array are compared using the strict comparison operator, which does not account for `NaN` self-inequality.
 
 ```js
 const allEqual = arr => arr.every(val => val === arr[0]);