Add PR as rule documentation

Co-authored-by: Marco Zühlke <[email protected]>

Author: satabin

modules/core/src/main/resources/scalafix-migrations.conf

@@ -93,7 +93,8 @@ migrations = [
     groupId: "org.gnieh,
     artifactIds: ["fs2-data-json.*"],
     newVersion: "1.8.0",
-    rewriteRules: ["github:gnieh/fs2-data/json-parse?sha=v1.8.0"]
+    rewriteRules: ["github:gnieh/fs2-data/json-parse?sha=v1.8.0"],
+    doc: "https://github.com/scala-steward-org/scala-steward/pull/3123"
   },
   {
     groupId: "org.http4s",