tweak unbound record field error message

jscomp/build_tests/super_errors/expected/unbound_record_field.res.expected

@@ -0,0 +1,16 @@
+
+  We've found a bug for you!
+  /.../fixtures/unbound_record_field.res:3:6-21
+
+  1 │ let foo = x =>
+  2 │   switch x {
+  3 │   | {someUnknownField: 123} => ()
+  4 │   }
+  5 │ 
+
+  someUnknownField refers to a record field, but the record type it belongs to couldn't be found automatically.
+
+  If it's defined in another module or file, bring it into scope by:
+  - Prefixing it with said module name: TheModule.someUnknownField
+  - Or specifying its type:
+  let theValue: TheModule.theType = {someUnknownField: VALUE}

jscomp/build_tests/super_errors/fixtures/unbound_record_field.res

@@ -0,0 +1,4 @@
+let foo = x =>
+  switch x {
+  | {someUnknownField: 123} => ()
+  }

jscomp/ml/typetexp.ml

@@ -910,7 +910,7 @@ let report_error env ppf = function
   | Unbound_label lid ->
     (* modified *)
     Format.fprintf ppf "@[<v>\
-                        @{<info>The record field %a can't be found.@}@,@,\
+                        @{<info>%a@} refers to a record field, but the record type it belongs to couldn't be found automatically.@,@,\
                         If it's defined in another module or file, bring it into scope by:@,\
                         @[- Prefixing it with said module name:@ @{<info>TheModule.%a@}@]@,\
                         @[- Or specifying its type:@ @{<info>let theValue: TheModule.theType = {%a: VALUE}@}@]\