Minor 
Fails against automated checks. 
Error: the repository field is empty. Nothing was checked.Since the SMS dev docs were written, some additions and changes have been made, so we need to review their content and update/add to them as necessary.
The page to be reviewed and updated in this task is https://moodledev.io/docs/4.5/apis/subsystems/sms (Guides > API Guides > Subsystems > SMS API)
We should take a couple of hours at least to review this in detail, but timebox it to less than a day.
The scope of this issue is limited to the dev docs. The user docs are covered by a separate issue.
