Clarify that route defaults doesn't need a route placeholder

[wouterj]

Let's move some feedback given in the DX issues to the docs.

In symfony/symfony#11171 , it was made clear that people don't get clear where defaults can be used for. The documentation makes it look like it's only used to have default values for route placeholders. It turns out, you can use it for a lot more things (it's an array of route specific settings). One common used example is the _controller default.

I would suggest to add a small paragraph (or a .. sidebar:: or .. tip::) about the other uses of the defaults and how you can retrieve them from the route attributes of a route.

[iamdto]

I'm willing to work on this but, isn't this already discussed in the Controller chapter? What specific point should we talk about? Or is this just a reminder and a clarification on of how you can use defaults in your controller?

[stof]

@iamdto The clarification needed is that you can use some defaults for attributes which don't appear in the URL (in which case the final version of the attribute will always be the default value).

[xabbuh]

I'm going to close this as #4017 has been merged.