[system_requirements] How to consume system requirements only when building a package

[uilianries]

Related to conan-io/conan#16625 and #3974

The Conan documentation could have those cases reported when system package managers should execute only when build the package, not when installing with pre-built binaries.

The issue original issue has a basic explanation, but could be integrated to the official documentation, with a better view of the recipe example.