Clarify shader language version(s) in manual

Do we need clarification around what shader language version we support?

From https://defold.com/manuals/shader/:
```
Specifications of OpenGL ES 2.0 (OpenGL for Embedded Systems) and OpenGL ES Shading Language can be found at https://www.khronos.org/registry/gles/

Observe that on desktop computers it is possible to write shaders using features not available on OpenGL ES 2.0. Your graphics card driver may happily compile and run shader code that will not work on mobile devices.
```

A user mentioned that we support GLES 2, but in truth, we're moving away from it, as it's only a fallback on Android.