Add missing PHPDoc annotation for Doctrine Cache implementations

This fixes the following deprecations produced by Symfony:
* User Deprecated: Method "Doctrine\Common\Cache\Cache::fetch()" might add "mixed" as a native return type declaration in the future. Do the same in implementation "Aws\DoctrineCacheAdapter" now to avoid errors or add an explicit @return annotation to suppress this message.
* User Deprecated: Method "Doctrine\Common\Cache\Cache::save()" might add "bool" as a native return type declaration in the future. Do the same in implementation "Aws\DoctrineCacheAdapter" now to avoid errors or add an explicit @return annotation to suppress this message.
* User Deprecated: Method "Doctrine\Common\Cache\Cache::delete()" might add "bool" as a native return type declaration in the future. Do the same in implementation "Aws\DoctrineCacheAdapter" now to avoid errors or add an explicit @return annotation to suppress this message.
* User Deprecated: Method "Doctrine\Common\Cache\Cache::contains()" might add "bool" as a native return type declaration in the future. Do the same in implementation "Aws\DoctrineCacheAdapter" now to avoid errors or add an explicit @return annotation to suppress this message.
* User Deprecated: Method "Doctrine\Common\Cache\Cache::getStats()" might add "?array" as a native return type declaration in the future. Do the same in implementation "Aws\DoctrineCacheAdapter" now to avoid errors or add an explicit @return annotation to suppress this message.

.changes/nextrelease/enhancement-add-phpdoc-doctrine-cache

@@ -0,0 +1,7 @@
+[
+    {
+        "type": "enhancement",
+        "category": "",
+        "description": "Add missing PHPDoc annotation for Doctrine Cache implementations."
+    }
+]

src/DoctrineCacheAdapter.php

@@ -18,6 +18,9 @@ public function get($key)
         return $this->cache->fetch($key);
     }
 
+    /**
+     * @return mixed
+     */
     public function fetch($key)
     {
         return $this->get($key);
@@ -28,6 +31,9 @@ public function set($key, $value, $ttl = 0)
         return $this->cache->save($key, $value, $ttl);
     }
 
+    /**
+     * @return bool
+     */
     public function save($key, $value, $ttl = 0)
     {
         return $this->set($key, $value, $ttl);
@@ -38,16 +44,25 @@ public function remove($key)
         return $this->cache->delete($key);
     }
 
+    /**
+     * @return bool
+     */
     public function delete($key)
     {
         return $this->remove($key);
     }
 
+    /**
+     * @return bool
+     */
     public function contains($key)
     {
         return $this->cache->contains($key);
     }
 
+    /**
+     * @return mixed[]|null
+     */
     public function getStats()
     {
         return $this->cache->getStats();