Better documentation for transitive header and transitive libs (traits)

When looking for `transitive_headers` and `transitive_libs` parameter for [requirements](https://docs.conan.io/2/reference/conanfile/methods/requirements.html#requirements), I can find:

- https://docs.conan.io/2/reference/conanfile/methods/requirements.html#transitive-headers
- https://docs.conan.io/2/reference/conanfile/methods/requirements.html#transitive-libs

Still, the documentation only specifies the WHAT, not the WHY, or recommended cases.

Right now, the [requirements traits](https://github.com/conan-io/tribe/blob/main/design/026-requirements_traits.md) design is more informative, and could be used as inspiration.

It would be great if we have a better explanation, like:

- Transitive headers are useful when a public header of a project has included a downstream header, resulting that the consumer will need to have access to both upstream and downstream headers