Unwrap fill with single part in cleanDoc (#13350)

tests/integration/__tests__/doc-utilities-clean-doc.js

@@ -14,7 +14,7 @@ describe("cleanDoc", () => {
     [
       "fill",
       [fill([""]), fill([]), fill(["1"]), fill(["2", line, "3"])],
-      [fill(["1"]), fill(["2", line, "3"])],
+      ["1", fill(["2", line, "3"])],
     ],
     ["nested group", group(group("_")), group("_")],
     [