Improve BacklinkCache field docs

Change-Id: I8303d5577ac84b8882e5ad79ac3058d6bf49356f

includes/cache/BacklinkCache.php

@@ -53,6 +53,7 @@ class BacklinkCache {
 	 * @see BacklinkCache::partitionResult()
 	 *
 	 * Cleared with BacklinkCache::clear()
+	 * @var array[]
 	 */
 	protected $partitionCache = array();
 
@@ -62,6 +63,7 @@ class BacklinkCache {
 	 *
 	 * Initialized with BacklinkCache::getLinks()
 	 * Cleared with BacklinkCache::clear()
+	 * @var ResultWrapper[]
 	 */
 	protected $fullResultCache = array();