HTML API: Fix punctuation in serialize_token() docblock.

Adds commas to separate the conditions in the compound conditional describing when an empty string is returned.

src/wp-includes/html-api/class-wp-html-processor.php

@@ -1340,7 +1340,7 @@ public function serialize(): ?string {
 	 * Serializes the currently-matched token.
 	 *
 	 * This method produces a fully-normative HTML string for the currently-matched token,
-	 * if able. If not matched at any token or if the token doesn't correspond to any HTML
+	 * if able. If not matched at any token, or if the token doesn't correspond to any HTML,
 	 * it will return an empty string (for example, presumptuous end tags are ignored).
 	 *
 	 * @see static::serialize()