Hi,
since the 2.6 API docs on www.gtk.org seems to not be available anymore, I put them on http://geany.uvena.de/manual/gtk/ for reference. These docs are meant for people who want to write plugins for Geany or want to modify Geany's source code.
Obviously you can also use the current API docs for these libraries. But using the 2.6 docs of the GTK libs (including GLib, GDK and Pango) has the advantages that you don't get confused by any newer API additions and you don't have to take care about whether you can use them or not. This is because Geany depends on GTK 2.6. API symbols from newer GTK/GLib versions should be avoided to keep the source code building against GTK 2.6. Therefore the offer of the outdated docs.
Plugins authors doesn't have to keep their plugins compatible with GTK 2.6, but maybe they want to because Geany does.
To make a long story short: if anyone wants to use GTK 2.6 docs, you can find them on http://geany.uvena.de/manual/gtk/ ;-).
Regards, Enrico