ENH member_from_x methods can return null

and fix extra return tag

src/Security/Member.php

@@ -575,8 +575,7 @@ class Member extends DataObject
      * @param string $hash The hash key
      * @param bool $login Should the member be logged in?
      *
-     * @return Member the matching member, if valid
-     * @return Member
+     * @return Member|null the matching member, if valid or null
      */
     public static function member_from_autologinhash($hash, $login = false)
     {
@@ -597,7 +596,7 @@ class Member extends DataObject
      * Find a member record with the given TempIDHash value
      *
      * @param string $tempid
-     * @return Member
+     * @return Member|null the matching member, if valid or null
      */
     public static function member_from_tempid($tempid)
     {