make wording about stateful filters less strong

Author: Narretz

docs/content/guide/filter.ngdoc

@@ -35,12 +35,17 @@ E.g. the markup `{{ 1234 | number:2 }}` formats the number 1234 with 2 decimal p
 
 In templates, filters are only executed when their inputs have changed. This is more performant than executing
 a filter on each {@link ng.$rootScope.Scope#$digest `$digest`} as is the case with {@link guide/expression expressions}.
-However, this is only true for filters that take [primitive values](https://developer.mozilla.org/docs/Glossary/Primitive)
+
+There are two exceptions to this rule:
+
+1. In general, this applies only to filters that take [primitive values](https://developer.mozilla.org/docs/Glossary/Primitive)
 as inputs. Filters that receive [Objects](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Objects)
-as input are executed on each `$digest`, as it would be too costly to track if they have changed.
+as input are executed on each `$digest`, as it would be too costly to track if the inputs have changed.
+
+2. Filters that are marked as `$stateful` are also executed on each $digest.
+See {@link guide/filters#stateful-filters Stateful filters} for more information. Note that no Angular
+core filters are $stateful.
 
-This means custom filters should only rely on their inputs and not on external state (e.g. other Angular services).
-See {@link guide/filter#creating-custom-filters Creating custom filters} below for more information.
 
 ## Using filters in controllers, services, and directives
 
@@ -103,8 +108,9 @@ as the first argument. Any filter arguments are passed in as additional argument
 function.
 
 The filter function should be a [pure function](http://en.wikipedia.org/wiki/Pure_function), which
-means that it should be stateless and idempotent. Angular relies on these properties and executes
-the filter only when the inputs to the function change.
+means that it should be stateless and idempotent, and not rely for example on other Angular services.
+Angular relies on this contract and will by default execute a filter only when the inputs to the function change.
+{@link guide/filters#stateful-filters Stateful filters} are possible, but less performant.
 
 <div class="alert alert-warning">
 **Note:** Filter names must be valid angular {@link expression} identifiers, such as `uppercase` or `orderBy`.