Add explanation for GHC-97170.

Author: kosmikus

message-index/messages/GHC-97170/doubleRoleAnnotationOnData/after/DoubleRoleAnnotationOnData.hs

@@ -0,0 +1,5 @@
+{-# LANGUAGE RoleAnnotations #-}
+module DoubleRoleAnnotationOnData where
+
+data Foo a = MkFoo a
+type role Foo nominal

message-index/messages/GHC-97170/doubleRoleAnnotationOnData/before/DoubleRoleAnnotationOnData.hs

@@ -0,0 +1,6 @@
+{-# LANGUAGE RoleAnnotations #-}
+module DoubleRoleAnnotationOnData where
+
+data Foo a = MkFoo a
+type role Foo nominal
+type role Foo representational

message-index/messages/GHC-97170/doubleRoleAnnotationOnData/index.md

@@ -0,0 +1,18 @@
+---
+title: Double role annotation on datatype
+---
+
+## Error Message
+
+```
+DoubleRoleAnnotationOnData.hs:6:1: error: [GHC-97170]
+    Duplicate role annotations for ‘Foo’:
+      type role Foo nominal
+          -- written at DoubleRoleAnnotationOnData.hs:5:1-21
+      type role Foo representational
+          -- written at DoubleRoleAnnotationOnData.hs:6:1-30
+  |
+6 | type role Foo representational
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+```
+

message-index/messages/GHC-97170/index.md

@@ -0,0 +1,11 @@
+---
+title: Duplicate role annotations
+summary: A type declaration has more than one accompanying role annotation
+severity: error
+introduced: 9.8.1
+---
+
+This error occurs if more than one role annotation occurs for a given datatype.
+Note that the role annotations need not be conflicting for this error to arise.
+Also note that the role annotations can occur in different locations from the
+datatype.