Fixed some doc problems

src/pocketmine/command/Command.php

@@ -139,7 +139,7 @@ abstract class Command{
 	}
 
 	/**
-	 * @return string
+	 * @return string|null
 	 */
 	public function getPermission(){
 		return $this->commandData["pocketminePermission"] ?? null;

src/pocketmine/command/CommandReader.php

@@ -160,7 +160,7 @@ class CommandReader extends Thread{
 	 */
 	public function getLine(){
 		if($this->buffer->count() !== 0){
-			return $this->buffer->shift();
+			return (string) $this->buffer->shift();
 		}
 
 		return null;

src/pocketmine/command/defaults/ParticleCommand.php

@@ -133,7 +133,7 @@ class ParticleCommand extends VanillaCommand{
 	 * @param float    $zd
 	 * @param int|null $data
 	 *
-	 * @return Particle
+	 * @return Particle|null
 	 */
 	private function getParticle($name, Vector3 $pos, $xd, $yd, $zd, $data){
 		switch($name){