docs: Document defer task options

[mjpieters]

• Fix the defer_call JSON schema to include the vars mapping.
• Update the schema reference documentation to cover defer: {task: ..., vars: {...}}.

[mjpieters]

Do let me know if #1879 is likely to be accepted, as that would require updates to this PR (we can then simplify the JSON schema to use "anyOf": [{"type": "string"},{"$ref": "#/definitions/task_call"}], and we could retool [`Dependency`](#dependency) in the schema reference documentation to work for both dependencies and deferred tasks).