I'm unsure why you call this undocumented. On the linked page "tex" is listed as built-in function and there for documented (kind-of).
Naturally, documentation is very thin for many topics. And more, more, more (!) examples, ideally combined in one place would be helpful. :)
If you really search for undocumented commands you can still check the source code on git-hub. I.e.:
https://github.com/godotengine/godot/blob/master/drivers/gles2/shader_compiler_gles2.cpp
...for the shader language.
