[Authflow] Remarks on the documentation of Authentication Flow API

- [ ] Mention phone number is expected to be in E.164 format with only `+` and digits, no separators nor spaces are allowed.
- [ ] Document all possible steps, their data, and their expected input.

Locate the Authentication Flow API documentation and the definition of its possible steps. Verify the phone-number requirement and document every step, its data, and expected input; done means both checklist items are covered accurately.

Written by the indexing model from the issue text.