porting some help texts from simple cms to the routing

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

this help text was originally in SimpleCmsBundle but would be just as useful here. maybe we can change the admin of SimpleCmsBundle after merging this to translate those fields in the CmfRoutingBundle translation domain to avoid duplicating such help texts.