Auto merge of #115914 - GuillaumeGomez:turn-custom_code_classes_in_docs-into-warning, r=Manishearth

Turn custom code classes in docs into warning

By habit, since it was a new feature gate, I added a check which emitted an error in case the new syntax was used. However, since rustdoc tags parser was accepting *everything*, using the "new" syntax should never ever emit errors. It now emits a warning.

Follow-up of #110800.

cc `@Manishearth`
r? `@notriddle`

Author: bors