better words on the hcl2 merge

website/pages/docs/from-1.5/blocks/build/source.mdx

@@ -24,6 +24,7 @@ build {
 }
 ```
 
--> **Note:** If any top-level attribute is re-defined with the same name an
-error will be displayed. For example: you cannot define the `output` field in
-the source declaration **and** in the source usage.
+-> **Note:** It is **not allowed** to set the same field in a top-level source
+  block and in a used source block. For example, if in the above example, the
+  top-level "amazon-ebs.example" source block also had an `output` field;
+  Packer would error.

website/pages/partials/from-1.5/builds/example-block.mdx

@@ -12,7 +12,7 @@ build {
         # Use the singular `source` block set
         # specific fields.
         # Note that fields cannot be overwritten, in other words, you cannot 
-        # set output from the top-level source block and here.
+        # set the 'output' field from the top-level source block and here.
         output = "different value"
         name = var.foo
     }