Move on_unimplemented syntax rule to be first

Author: ehuss

src/attributes/diagnostics.md

@@ -514,10 +514,6 @@ The *`diagnostic::on_unimplemented` [attribute][attributes]* is a hint to the co
 >    = note: Note 2
 > ```
 
-
-r[attributes.diagnostic.on_unimplemented.allowed-positions]
-The attribute should be placed on a [trait declaration], though it is not an error to be located in other positions.
-
 r[attributes.diagnostic.on_unimplemented.syntax]
 The attribute uses the [MetaListNameValueStr] syntax to specify its inputs, though any malformed input to the attribute is not considered as an error to provide both forwards and backwards compatibility.
 
@@ -527,6 +523,9 @@ The following keys have the given meaning:
 * `label` --- The text for the label shown inline in the broken code in the error message.
 * `note` --- Provides additional notes.
 
+r[attributes.diagnostic.on_unimplemented.allowed-positions]
+The attribute should be placed on a [trait declaration], though it is not an error to be located in other positions.
+
 r[attributes.diagnostic.on_unimplemented.note-repetition]
 The `note` option can appear several times, which results in several note messages being emitted.