Document the minimum scope of configuration options since not all of them can be applied at partition-level

[vkarak]

This is a follow up from #3402. Copying here the detailed explanation:

not all configuration options have an effect at partition level, since many of them affect global aspects, such as the database file. Switching system partitions (and therefore configuration options) is something that happens only during test execution, so only options that are related to tests (e.g., general/trap_job_errors, general/use_login_shell etc.) will "understand" partition scope changes.

The --show-config option does not know if the requested option has a system or partition-specific effect, so it simply performs the resolution based on the --system option and the target_systems scope.

Documenting the scope of all configuration options and maybe adding metadata to help --show-config show always the right value probably deserves a separate issue. I will open one.

We should also highlight the current limitation of --show-config.