Update phpdoc annotations

Author: tuyakhov

src/ResourceIdentifierInterface.php

@@ -5,9 +5,20 @@
 
 namespace tuyakhov\jsonapi;
 
+/**
+ * Interface for a “resource identifier object” that identifies an individual resource.
+ */
 interface ResourceIdentifierInterface
 {
+    /**
+     * The "id" member of a resource object.
+     * @return string an ID that in pair with type uniquely identifies the resource.
+     */
     public function getId();
 
+    /**
+     * The "type" member of a resource object.
+     * @return string a type that identifies the resource.
+     */
     public function getType();
 }

src/ResourceInterface.php

@@ -5,10 +5,22 @@
 
 namespace tuyakhov\jsonapi;
 
+/**
+ * Interface for a “resource object” that represent an individual resource.
+ */
 interface ResourceInterface extends ResourceIdentifierInterface
 {
+    /**
+     * The "attributes" member of the resource object representing some of the resource’s data.
+     * @param array $fields specific fields that a client has requested.
+     * @return array an array of attributes that represent information about the resource object in which it’s defined.
+     */
     public function getResourceAttributes(array $fields = []);
 
+    /**
+     * The "relationships" member of the resource object describing relationships between the resource and other JSON API resources.
+     * @return ResourceIdentifierInterface[] represent references from the resource object in which it’s defined to other resource objects.
+     */
     public function getResourceRelationships();
 
     public function setResourceRelationship($name, $relationship);

src/ResourceTrait.php

@@ -14,14 +14,11 @@
 trait ResourceTrait
 {
     /**
-     * @return null|string
+     * @return string
      */
     public function getId()
     {
-        if ($this instanceof ActiveRecordInterface) {
-            return (string) $this->getPrimaryKey();
-        }
-        return null;
+        return (string) ($this instanceof ActiveRecordInterface ? $this->getPrimaryKey() : null);
     }
 
     /**

tests/data/ResourceModel.php

@@ -29,7 +29,7 @@ class ResourceModel extends Model implements ResourceInterface, LinksInterface
 
     public function getId()
     {
-        return static::$id;
+        return (string) static::$id;
     }
 
     public function fields()