Cleanup left over set-to-global-defaults description

In #518750: Rethink the system settings form, the "reset to defaults" button was removed from the theme settings forms on /admin/appearance/settings (global settings) and /admin/appearance/settings/[themename] (theme-specific settings).

Unfortunately, it isn't clear to the user how the global settings relate to the theme-specific settings. Theme-specific settings are following the global settings 'automagically' until the first time you save the settings form for a specific theme. On that moment, that theme stops following the global settings and starts storing it's own settings. I think the theme settings forms are suffering from a couple of problems:
1) The user doesn't know when he is breaking the invisible link with the global settings.
2) The theme even starts using its own settings (instead of global) if the form is submitted without changes.
3) On the theme-specific form, you can't see if you're looking at default values (loaded from the global settings) or at theme-specific settings.
4) There is no way to switch back to global settings (but OK, that choice was made in #518750).
5) It is not clear is global settings are overridden per-setting (which is not the case) or for the theme as a whole (which is what happens).
6) The help text still refers to the "reset to default" button which does exist anymore.

Maybe I'm missing a really important use case here, but I don't see the need for global theme settings at all. You enable a theme, configure the settings and you're good to go. I only do that once or twice on each site I build. The global settings are making an educated guess as long as I haven't adjusted the theme-specific settings. That's not a huge time saver, especially now that the "Display author and date information" settings have been moved elsewhere. As far as I'm concerned, default values can simply be hard-coded in variable_get calls, just like you would do with your average module.

I'm afraid that it's too late to address all of this in D7, except for point 6), so for now, this patch only fixes the incorrect help text. If it's not yet too late to fix other points as well, please let me know so we can work on the patch some more.