docs: Documented OAuth2Client.start

Author: mccoderpy

discord/oauth2/client.py

@@ -350,6 +350,13 @@ def setup_logging(
                     )
 
     async def start(self) -> Self:
+        """|coro|
+        
+        Starts the client. This will start the internal HTTP client and call the :attr:`on_startup` event.
+        
+        .. important::
+            This must be called before any other requests are made.
+        """
         self.setup_logging()
 
         if not self.loop: