Lack of documentation for custom GTK3 widgets in C

[Drew DeVault <sir cmpwn com>]

I've found a reasonable amount of resources online for making custom
GTK2 widgets in C. Unfortunately, I can't say the same about GTK3. Can
anyone point me to some resources for this, and can we do something
about the gap in documentation?

Note: gtk-devel in the CC because the mailing list page indicates that
documentation improvements should be discussed on it. Apologies if this
is in error.

--
Drew DeVault