Add example for creating a signature, and update the example for crea…

…ting an attendee via the API to make clearer how to do it

source/includes/authenticated_api/_attendees.md.erb

@@ -146,11 +146,11 @@ Find an attendee by email address. Once you have obtained the attendee's id you
     },
     "data_processing_consent_type": "explicit",
     "email_opt_in_type": {
-      "context": "web_form",
-      "kind": "radio_sure",
+      "context": "external",
+      "kind": "external",
       "mailable": true,
-      "external_id": null,
-      "active": true
+      "external_id": "canvass-signature-2022",
+      "active": false
     },
     "member": {
       "id": 1124,

source/includes/authenticated_api/_signatures.md.erb

@@ -132,6 +132,22 @@ The JSON response has a single "signature" object, which may include the followi
 <div></div>
 
 ### Create
+> POST /api/v1/petitions/no-taxes-on-tea/signatures/
+
+```json
+{
+  "signature": {
+    "first_name": "Foo",
+    "last_name": "Bar",
+    "email": "[email protected]",
+    "postcode": "10010",
+    "phone_number": "123-456",
+    "country": "US",
+    "email_opt_in_type_external_id": "abcd12345",
+    "join_organisation": true
+  }
+}
+```
 
 > Response body
 
@@ -171,11 +187,11 @@ The JSON response has a single "signature" object, which may include the followi
     "data_processing_consent_type": "explicit",
     "partnership_opt_ins": [],
     "email_opt_in_type": {
-      "context": "web_form",
-      "kind": "radio_sure",
+      "context": "external",
+      "kind": "external",
       "mailable": true,
-      "external_id": null,
-      "active": true
+      "external_id": "abcd12345",
+      "active": false
     },
     "member": {
       "id": 111222333,