emit "DC_EVENT_MEMBER_ADDED" and python plugin event "member_added" for securejoin or non-securejoin additions of a contact to a chat. also fixup some docs

src/chat.rs

@@ -1958,6 +1958,10 @@ pub(crate) fn add_contact_to_chat_ex(
             chat_id,
             msg_id: msg.id,
         });
+        context.call_cb(Event::MemberAdded {
+            chat_id,
+            contact_id: contact.id,
+        });
     }
     context.call_cb(Event::MsgsChanged {
         chat_id,