dot/gtk3-settings.ini: Add reminder about Gtk 3's vanishing scrollbars.

[profile] / dot / gtk3-settings.ini

[Settings]
gtk-primary-button-warps-slider = false
gtk-theme-name = Adwaita-dark
gtk-application-prefer-dark-theme = true
gtk-long-press-time = 5000
#gtk-overlay-scrolling = false