Add missing comment

[janek109]

No description provided.

[janek109]

This comment will help find where example code should be pasted

[javiereguiluz]

@janek109 thanks for your first Symfony Docs contribution! These comments are very important for us, so thanks for adding this missing comment.

I see that you sent this pull request to "master" branch. It's OK, but for the next pull requests, please create them in "the oldest maintained branch that contains the problem". At the moment we maintain 2.7, 2.8, 3.3, 3.4 and master ... so we probably should merge this in 2.7. For this pull request you don't have to worry and you don't have to create another one, because the doc mergers will change the branch while merging ... but for complex pull requests is much more difficult to do it, so it's better to use the right branch from the beginning. Thanks!

[xabbuh]

Thank you @janek109.