Make Cmd::to_command public and add docs

[domenicquirl]

This is intended to increase discoverability of the xshell::Cmd -> std::process::Command conversion, which is already accessible via a From implementation which internally forwards directly to to_command, and be able to document which builder methods translate to the new command and which only apply when executing the xshell version (e.g., quiet obviously doesn't apply, but ignore_status and stdin also don't, since their effects only happen once the command is invoked).