Better doc about overriding third-party translations

[stof]

Translations are not tied to a bundle, but to a translation domain. So overriding translation is done by putting translations in the same domain (with the same key) in translations/.

This needs to be documented explicitly, as even some core team members were thinking that it was more complex than that.

[yceruto]

I think that's already clear in this section http://symfony.com/doc/4.0/bundles/override.html#translations

[yceruto]

And the translations/ path is mentioned here:

• http://symfony.com/doc/4.0/best_practices/i18n.html#translation-source-file-location,
• http://symfony.com/doc/4.0/configuration/override_dir_structure.html,
• http://symfony.com/doc/4.0/translation.html#translation-resource-file-names-and-locations

[stof]

Well, as you can see from the issue reported by @javiereguiluz in the code repo, the existing doc might not be clear enough.

[javiereguiluz]

Fixed by #9935.