Merge changes Ic4c0597a,I1b87b3a4,Id2fdf52b into main

* changes:
  Reword/reorder the bit about enums without backing types to make it obvious that new code shouldn't do this.
  Improve some wording in the callbacks section.
  Expand and improve the section about error handling.
tree: a87a9438994c6a9d02a90b5ad21eb79602d5cf5e
  1. api-guidelines/