[docs] return value for chainable methods

Author: yyx990803

docs/en/api/after-each.md

@@ -12,6 +12,10 @@ You can add multiple global after hooks to the same router. These hooks will be
 
   The hook function receives a single argument which is a [Transition Object](../pipeline/hooks.html#transition-object), but you can only access its `to` and `from` properties, which are route objects. You **cannot** call transition methods in the global after hook.
 
+### Returns
+
+- The router instance itself.
+
 ### Example
 
 ``` js

docs/en/api/alias.md

@@ -10,6 +10,10 @@ For example, if we alias `/a` to `/a/b/c`, when we visit `/a`, the browser URL w
 
   The alias map object should be in the form of { fromPath: toPath, ... }. The paths can contain dynamic segments.
 
+### Returns
+
+- The router instance itself.
+
 ### Example
 
 ``` js

docs/en/api/before-each.md

@@ -10,6 +10,10 @@ You can add multiple global before hooks to the same router. These hooks will be
 
   The hook function receives a single argument which is a [Transition Object](../pipeline/hooks.html#transition-object).
 
+### Returns
+
+- The router instance itself.
+
 ### Example
 
 Basic

docs/en/api/map.md

@@ -8,6 +8,10 @@ The main method to define route mappings for the router.
 
   An object whose keys are paths and values are route config objects. For path matching rules, see [Route Matching](../route.html#route-matching).
 
+### Returns
+
+- The router instance itself.
+
 ### Route Config Object
 
 A route config object can contain two fields:

docs/en/api/on.md

@@ -7,6 +7,10 @@ Add a single root-level route configuration. Internally, `router.map()` simply c
 - `path: String` - see [Route Matching](../route.md#route-matching)
 - `config: Object` - see [Route Config Object](map.md#route-config-object).
 
+### Returns
+
+- The router instance itself.
+
 ### Example
 
 ``` js

docs/en/api/redirect.md

@@ -8,6 +8,10 @@ Configures global redirection rules for the router. Global redirections are perf
 
   The redirect map object should be in the form of `{ fromPath: toPath, ... }`. The paths can contain dynamic segments.
 
+### Returns
+
+- The router instance itself.
+
 ### Example
 
 ``` js