Add some comments

src/Fields/fieldvector.jl

@@ -320,6 +320,10 @@ end
     parent(getfield(_values(fv), symb))
 @inline transform_broadcasted(x, symb, axes) = x
 
+# We pass `ToCPU` so that we can define unambiguous
+# and recursion-friendly `copyto_per_field!` methods
+# we don't use `ToCPU`, so its value doesn't matter.
+
 @inline Base.copyto!(
     dest::FieldVector,
     bc::Union{FieldVector, Base.Broadcast.Broadcasted{FieldVectorStyle}},
@@ -350,7 +354,10 @@ for dev in (:ToCPU, :ToCUDA)
     end
 end
 
-# Need
+# We need multiple methods here to avoid ambiguities
+# Also, we pass `ToCPU` so that we can define unambiguous
+# and recursion-friendly `copyto_per_field_scalar!` methods
+# we don't use `ToCPU`, so its value doesn't matter.
 
 @inline Base.copyto!(
     dest::FieldVector,