docs: add return types in sample code

Author: kenjis

user_guide_src/source/outgoing/view_cells/014.php

@@ -12,7 +12,7 @@ class AlertMessageCell extends Cell
     protected $message;
     private $computed;
 
-    public function mount()
+    public function mount(): void
     {
         $this->computed = sprintf('%s - %s', $this->type, $this->message);
     }

user_guide_src/source/outgoing/view_cells/016.php

@@ -10,7 +10,7 @@ class RecentPostsCell extends Cell
 {
     protected $posts;
 
-    public function linkPost($post)
+    public function linkPost($post): string
     {
         return anchor('posts/' . $post->id, $post->title);
     }

user_guide_src/source/outgoing/view_cells/018.php

@@ -8,7 +8,7 @@ class RecentPostsCell extends Cell
 {
     protected $posts;
 
-    public function mount()
+    public function mount(): void
     {
         $this->posts = model('PostModel')->orderBy('created_at', 'DESC')->findAll(10);
     }

user_guide_src/source/outgoing/view_cells/019.php

@@ -10,7 +10,7 @@ class RecentPostsCell extends Cell
 {
     protected $posts;
 
-    public function mount(?int $categoryId)
+    public function mount(?int $categoryId): void
     {
         $this->posts = model('PostModel')
             ->when(