Improve comments.

Author: Crell

src/EnvMapper.php

@@ -99,6 +99,10 @@ private function typeNormalize(string $val, \ReflectionProperty $rProp): int|flo
     /**
      * This is actually rather slow.  Reflection's performance cost hurts here.
      *
+     * We only care about defaults from the constructor; if a non-readonly property
+     * has a default value, then newInstanceWithoutConstructor() will use it for us
+     * and we don't need to do anything.
+     *
      * @param \ReflectionProperty $subject
      * @return mixed
      */