[TASK] Mark OutputFormat::nextLevel() as @internal

This method is used internally for rendering the CSS, and it is
not intended to be called from outside this library.

Author: oliverklee

CHANGELOG.md

@@ -21,6 +21,7 @@ Please also have a look at our
 
 ### Changed
 
+- Mark `OutputFormat::nextLevel()` as `@internal` (#901)
 - Only allow `string` for some `OutputFormat` properties (#885)
 - Make all non-private properties `@internal` (#886)
 - Use more native type declarations and strict mode

src/OutputFormat.php

@@ -780,6 +780,9 @@ public function indentWithSpaces(int $numberOfSpaces = 2): self
         return $this->setIndentation(\str_repeat(' ', $numberOfSpaces));
     }
 
+    /**
+     * @internal
+     */
     public function nextLevel(): self
     {
         if ($this->oNextLevelFormat === null) {