[d3d12] Document map_blend_factor (#4151)

wgpu-hal/src/dx12/conv.rs

@@ -222,6 +222,10 @@ pub fn map_polygon_mode(mode: wgt::PolygonMode) -> d3d12_ty::D3D12_FILL_MODE {
     }
 }
 
+/// D3D12 doesn't support passing factors ending in `_COLOR` for alpha blending
+/// (see https://learn.microsoft.com/en-us/windows/win32/api/d3d12/ns-d3d12-d3d12_render_target_blend_desc).
+/// Therefore this function takes an additional `is_alpha` argument
+/// which if set will return an equivalent `_ALPHA` factor.
 fn map_blend_factor(factor: wgt::BlendFactor, is_alpha: bool) -> d3d12_ty::D3D12_BLEND {
     use wgt::BlendFactor as Bf;
     match factor {