Resolve #3415995 "D10 symphony encoder"

Closes #3415995

src/Encoder/Doc.php

@@ -41,7 +41,7 @@ class Doc implements EncoderInterface {
   /**
    * {@inheritdoc}
    */
-  public function encode($data, $format, array $context = []) {
+  public function encode(mixed $data, string $format, array $context = []): string {
     switch (gettype($data)) {
       case 'array':
         // Nothing to do.
@@ -154,7 +154,7 @@ class Doc implements EncoderInterface {
   /**
    * {@inheritdoc}
    */
-  public function supportsEncoding($format) {
+  public function supportsEncoding(string $format): bool {
     return $format === static::$format;
   }