[Doc][Link] Clarify the documentation

[lyrixx]

Hello,

I read a lot the documentation about SubResource and Link https://api-platform.com/docs/core/subresources/ and I still don't understand how I'm supposed to use fromClass, toClass, fromProperty, and toProperty.

I tried many combinaison on my project, and I can't manage to make it work.

[soyuka]

Note that these only work with Doctrine ORM or Doctrine ODM using a path algorithm. Each links defines how the previous part of the URI links to the second. In your case you wanted to call another data source and this can't be automatic unless we call the provider of each path (which will lead to performance issues). So this is possible, but I'm not sure we should make it automatic. Let's follow this inside #7112