document route validation

[dbu]

document the validators added in symfony-cmf/routing-bundle#292

should go into the section "The PHPCR-ODM Route document" of https://github.com/symfony-cmf/symfony-cmf-docs/blob/master/bundles/routing/dynamic.rst, as subsection before talking about admins.

[dbu]

@EmmanuelVella it seems you ported the validation from sonata to the forms layer for the routing bundle. may i ask if you would have time to add documentation about them?

[EmmanuelVella]

@dbu Sure I will have a look at it.

[dbu]

👍

[dbu]

#694