Many small fixes to incomplete/unspecific/broken PHPDoc comments

E.g. adding missing types or fixing inconsistencies. This patch is
exclusively about comments and should be fairly easy to review, I
hope. It should be impossible such a patch can break anything.

Mistakes like this happen because the relevant MediaWiki CodeSniffer
sniff is disabled in this codebase. I would love to enable it at
least partially, if possible. This is done in preparation.

Change-Id: Ie6a676df1ac88eb65d1cb1b923e29d0e20d893e2

Author: thiemowmde

src/Entity/SerializableEntityId.php

@@ -10,6 +10,7 @@
  */
 abstract class SerializableEntityId implements EntityId {
 
+	/** @var string */
 	protected $serialization;
 
 	public const PATTERN = '/^[^:]+\z/';

src/Snak/PropertyValueSnak.php

@@ -19,6 +19,7 @@
  */
 class PropertyValueSnak extends SnakObject {
 
+	/** @var DataValue */
 	protected $dataValue;
 
 	/**