API.md

Previously some resource types were not validating requests for
uniqueness (the creation would have failed later, at database commit
time).

The availability API allows unauthenticated callers (i.e. everyone) to
query for existence of a specific resource, or resources that would
conflict with it in the global namespace. It's basically a public
validation check, meant so that applications that let users create
accounts can provide early feedback on availability.