Hi,
With allow_always_save=true, both Save and Save All are always enabled (unless there are no files, of course). Obviously, that makes sense for Save. But what you invoke Save All and there are no modified files? It doesn't save all files regardless of modification, as you may think, nor does it save the current file - in fact, it does nothing, and allow_always_save has no effect on it. So IMHO, en/disabling of Save All should not be affected by the value of allow_always_save.
The attached patch does that, and makes the preference changeable, provided that you call ui_save_buttons_toggle() after change (we'll need this when/if the various preferences editor is included in Geany). I also edited the documentation accordingly.
But that's enough description for a one-liner...