Add note on the requirement of paginate_by (#1403)

docs/content/documentation/templates/pagination.md

@@ -32,6 +32,8 @@ current_index: Number;
 total_pages: Number;
 ```
 
+**The variable will not be defined if `paginate_by` is not set to a positive number.**
+
 A pager is a page of the pagination; if you have 100 pages and paginate_by is set to 10, you will have 10 pagers each
 containing 10 pages.
 
@@ -76,4 +78,4 @@ Here is an example from a theme on how to use pagination on a page (`index.html`
         <a class="next" href="{{ paginator.next }}">Next ›</a>
     {% endif %}
 </nav>
-```
+```