Clarify and move the Sign-in text option

There is an option under Admin > Settings > General > Sign-in restriction > Sign-in text which purpose it is not very clear.

When restyle_login_page feature flag is enabled, the sign-in text appears below the sign-in boxes.

Should the intention of the Sign-in text be documented?
Why does only appear on the standard sign-in box? Should it appear on the others sign-in tabs?
Should the option be moved to Admin > Settings > Appearance > Sign in/Sign up pages?