Document currentWhen deprecation

source/deprecations/v2.x.html.md

@@ -44,6 +44,25 @@ let lastName = 'Doe';
 `Hello ${lastName}, ${firstName}`  // "Hello Doe, John"
 ```
 
+#### Link-to's `currentWhen`
+
+##### until: 3.0.0
+##### id: ember-routing-view.deprecated-current-when
+
+When developers wanted to customize in what route a `{{link-to}}` should be considered "active" they
+could pass it using `currentWhen`.
+
+```handlebars
+{{#link-to 'item' currentWhen='index'}}
+```
+
+Now `currentWhen` is deprecated in favour of the hyphenated `current-when`, with the exact
+same semantics:
+
+```handlebars
+{{#link-to 'item' current-when='index'}}
+```
+
 ### Deprecations Added in 2.1
 
 #### Initializer Arity