API: properly document error types

[Pikrass]

Exact URL where the bug happened

https://lishogi.org/api/challenge/open

Steps to reproduce the bug

Make a request while forgetting a required parameter

What did you expect to happen?

The documentation says the output should be an object of the form: {"error": "simple string"}.

What happened instead?

This is the output:

{
	"error": {
		"clock.increment": ["This field is required"]
	}
}