Document custom exceptions

Author: ArtyomVancyan

src/fastapi_oauth2/exceptions.py

@@ -2,16 +2,16 @@
 
 
 class OAuth2Error(HTTPException):
-    ...
+    """Base OAuth2 exception."""
 
 
 class OAuth2AuthenticationError(OAuth2Error):
-    ...
+    """Raised when authentication fails."""
 
 
 class OAuth2BadCredentialsError(OAuth2Error):
-    ...
+    """Raised when credentials are invalid."""
 
 
 class OAuth2InvalidRequestError(OAuth2Error):
-    ...
+    """Raised when request is invalid."""