Document the differences in normalized vertex positions in the backends

[kvark]

From the chat:

GL expects Z output in [-1,1], and that's what our math produces (cgmath catering to GL)
DX expects Z in [0,1](while still treating X and Y in [-1,1] like GL does), so we gotta patch it
note that Z we are patching is in the homogeneous coordinates, so we use W there, and the end result (Z/W) goes into [0,1]

Thus, for DX we need to replace this:

return mul(u_Transform, float4(pos));

by this:

float4 ndc = mul(u_Transform, float4(pos));
ndc.z = (ndc.z + ndc.w) * 0.5;
return ndc;

Related: #942

[Bastacyclop]

Maybe just describe them instead of describing their differences ?