docs: mention current lack of SIGTSTP support

[haosenwang1018]

Summary

• document that the TUI currently runs inside an alternate screen session and does not yet provide first-class SIGTSTP / Ctrl-Z suspend-resume handling
• recommend quitting and relaunching instead of relying on shell job control today
• answer the remaining suspend-support question in issue Brilliant! Comments for improvements #344 without pretending the feature already exists

Testing

• git diff --check