*/form_render: update an outdated comment

There is no fallback on group id, if no group title, no legend is
displayed
8 jobs for feat/form-group-legent in 0 seconds
Status Job ID Name Coverage
  External
passed ERP5.CodingStyleTest-TestRunner2

01:22:22

passed ERP5.UnitTest-TestRunner2

02:39:39

passed ERP5.CodingStyleTest-TestRunner2

01:35:44

passed ERP5.CodingStyleTest-TestRunner2

01:20:37

passed ERP5.CodingStyleTest-TestRunner2

00:45:44

failed ERP5.UnitTest-TestRunner2

03:22:39

failed ERP5.UnitTest-TestRunner2

03:15:15

failed ERP5.UnitTest-TestRunner2

01:44:38