Doc: Document cubemap and 3D texture uniforms for SkyMaterial

The uniform type table only listed TextureInput as sampler2D, but the
property tracker picks the sampler type from the referenced texture, so
a CubeMapTexture is exposed as samplerCube and 3D texture data as
sampler3D. The CubeMapSky baseline test already relies on this.

Document Texture as the texture property type and leave TextureInput
out, since new types using the property tracker are not meant to rely
on it. Also note that a texture property without a texture set is not
exposed to the shader at all, and merge the vector2d/3d/4d rows into
one row while at it.

Pick-to: 6.12
Assisted-by: Claude Code Fable 5.1
Change-Id: Idd6eeb2666cfaad10413ac3095ca98343a8b53f1
Reviewed-by: Christian Strømme <christian.stromme@qt.io>