[Routing] Add note on nuances of the Routing exclude config option

[toby-griffiths]

As per reported issue symfony/symfony#39588,
currently the exclude option for attribute routing only works if the
resource value is a string, and is a glob pattern, so this updates the
example to use a glob pattern, and advise of this, current nuance.

[javiereguiluz]

@toby-griffiths thanks for this contribution. This is indeed an annoying bug, so let's add a warning in the docs.

While merging I reworded this a bit (99427cb):

• Don't use a glob in the resource option to avoid confusion (we don't use it anywhere else)
• Extract the warning into a caution and move it outside of the YAML config example so it's more visible to readers

Thanks!