Update index.md (#11723)

* Update index.md

Better context.

* added a space

Co-authored-by: Estelle Weyl <estelle@openwebdocs.org>

files/en-us/web/javascript/reference/global_objects/array/lastindexof/index.md

@@ -35,7 +35,7 @@ lastIndexOf(searchElement, fromIndex)
     index is greater than or equal to the length of the array, the whole array will be
     searched. If negative, it is taken as the offset from the end of the array. Note that
     even when the index is negative, the array is still searched from back to front. If
-    the calculated index is less than 0, -1 is returned, i.e. the array will not be
+    the calculated index (`arr.length + fromIndex`) is less than 0, -1 is returned, i.e. the array will not be
     searched.
 
 ### Return value