Add documentation for the unsubscribe() MP API method

[MAILPOET-5152]

doc/api_methods/UnsubscribeGlobally.md

@@ -0,0 +1,29 @@
+[back to list](../Readme.md)
+
+# Unsubscribe from all lists and change subscriber status
+
+## `array unsubscribe(string $subscriber_id)`
+
+This method removes a subscriber from all lists and updates its status to 'unsubscribed'.
+
+It returns a subscriber. See [Get Subscriber](GetSubscriber.md) for returned data structure.
+
+## Arguments
+
+### string `$subscriber_id` (required)
+
+An id or email of an existing subscriber. An `\Exception` is thrown when an id or email doesn't match any subscriber.
+
+## Error handling
+
+All expected errors from the API are exceptions of class `\MailPoet\API\MP\v1\APIException`.
+Code of the exception is populated to distinguish between different errors.
+
+An exception of base class `\Exception` can be thrown when something unexpected happens.
+
+Codes description:
+
+| Code | Description                                  |
+| ---- | -------------------------------------------- |
+| 4    | Invalid subscriber that does not exist       |
+| 24   | Subscriber already has 'unsubscribed' status |