At the javadoc http://javadoc.lwjgl.org/ I can't find the Callbacks class. I used google and found it here http://javadoc.lwjgl.org/org/lwjgl/glfw/Callbacks.html
Why isn't it in the list of classes?
Thanks Evan407, this was indeed a bug and has been fixed. The CDN for the javadoc site caches data for up to a day (to reduce our costs), so give it some time and all missing classes will appear.
I also couldn't find this without google
it's like it's missing in the index and class lists.
The GLContext class has been removed in the latest 3.0.0b builds and will be removed from the CDN soon.
Please note that javadoc.lwjgl.org will always point to the javadoc applicable to the latest nightly build. After the 3.0.0 release, each new release will get a dedicated URL with the javadoc that is applicable to that release.