Enforcing unified support would be breaking

Author: christiangnrd

src/KernelAbstractions.jl

@@ -578,8 +578,9 @@ end
 Returns whether unified memory arrays are supported by the backend.
 
 !!! note
-    Backend implementations **must** implement this function
-    only if they **do** support unified memory.
+    Backend implementations **should** implement this function
+    only if they **do** support unified memory. It will be required
+    in KernelAbstractions 0.10.
 """
 supports_unified(::Backend) = false