[Docs]: "Installation root" and "app root" are vague terms and are not explained anywhere

### Feature or enhancement request details

The only way I was able to figure out what was meant by the terms "app root" and "installation root" is that **container system status** shows their current paths. From that, I infer that the installation root is "where the container project's binaries are installed" and that the app root is "everything else, including containers".

"Installation root" is ambiguous because the whole function of a container system is to install and run containers. Why would I even need to know or worry about **container**'s own internal installation root? I want to specify where _my_ stuff is going to be installed. Had I not seen the status output, I would have assumed that I needed to change the "installation root" to change where my containers would go.

"App root" is a similarly peculiar choice of term since **container** isn't "an app" as the term is conventionally used, and most containers don't contain "apps". My first guess at why there's an "app root" was that **container** did in fact install some sort of GUI interface, but clearly that's not right. My container setup is a completely app-free zone.

There doesn't seem to be any documentation for these terms. Even the **--help** output is unhelpful. Essentially it says that the **--install-root** option specifies where the install root is and the **--app-root** option specifies the app root. Thanks, **--help**, I'm glad we could have this little talk. 🙂

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