Added references to W3C Specification in docblocks

This is much better for understanding whats going on. Also as the reference links are pointing to 26/04/2018 version, it helps understanding if things have been changed etc.

src/MicrodataDOMDocument.php

@@ -10,6 +10,8 @@ class MicrodataDOMDocument extends \DOMDocument
     /**
      * Get top-level items of the document
      *
+     * @see https://www.w3.org/TR/2018/WD-microdata-20180426/#dfn-top-level-microdata-item
+     *
      * @return \DOMNodeList List of top level items as elements
      */
     public function getItems() : \DOMNodeList

src/MicrodataDOMElement.php

@@ -5,6 +5,8 @@ namespace YusufKandemir\MicrodataParser;
 class MicrodataDOMElement extends \DOMElement
 {
     /**
+     * @see https://www.w3.org/TR/2018/WD-microdata-20180426/#dfn-item-properties for details of algorithm
+     *
      * @return array
      */
     public function getProperties() : array
@@ -57,6 +59,8 @@ class MicrodataDOMElement extends \DOMElement
     }
 
     /**
+     * @see https://www.w3.org/TR/2018/WD-microdata-20180426/#dfn-property-name
+     *
      * @return array
      */
     public function getPropertyNames() : array
@@ -77,6 +81,8 @@ class MicrodataDOMElement extends \DOMElement
     }
 
     /**
+     * @see https://www.w3.org/TR/2018/WD-microdata-20180426/#dfn-property-value for details of algorithm
+     *
      * @return $this|string
      */
     public function getPropertyValue()
@@ -194,6 +200,8 @@ class MicrodataDOMElement extends \DOMElement
      *
      * @see \preg_split() for possible return values and behaviour
      *
+     * @see https://www.w3.org/TR/2018/WD-microdata-20180426/#dfn-split-a-string-on-spaces for definition of tokens
+     *
      * @param string $attribute
      *
      * @return array[]|false|string[]

src/MicrodataParser.php

@@ -73,6 +73,8 @@ class MicrodataParser
     }
 
     /**
+     * @see https://www.w3.org/TR/2018/WD-microdata-20180426/#dfn-get-the-object
+     *
      * @param MicrodataDOMElement $item
      * @param array $memory
      *