Clarified documentation; fixed broken link

Author: Bart Koelman

docs/usage/options.md

@@ -39,6 +39,9 @@ options.MaximumPageNumber = new PageNumber(50);
 options.IncludeTotalResourceCount = true;
 ```
 
+To retrieve the total number of resources on secondary and relationship endpoints, the reverse of the relationship must to be available. For example, in `GET /customers/1/orders`, both the relationships `[HasMany] Customer.Orders` and `[HasOne] Order.Customer` must be defined.
+If `IncludeTotalResourceCount` is set to `false` (or the inverse relationship is unavailable on a non-primary endpoint), best-effort paging links are returned instead. This means no `last` link and the `next` link only occurs when the current page is full.
+
 ## Relative Links
 
 All links are absolute by default. However, you can configure relative links.

docs/usage/resources/nullability.md

@@ -2,7 +2,7 @@
 
 Properties on a resource class can be declared as nullable or non-nullable. This affects both ASP.NET ModelState validation and the way Entity Framework Core generates database columns.
 
-ModelState validation is enabled by default since v5.0. In earlier versions, it can be enabled in [options](~/usage/options.md#enable-modelstate-validation).
+ModelState validation is enabled by default since v5.0. In earlier versions, it can be enabled in [options](~/usage/options.md#modelstate-validation).
 
 # Value types