Describe OAuthTokenError's client-side raise on the client OAuth page

OAuthTokenError is no longer only the token endpoint's rejection: a stored
client record whose token_endpoint_auth_method this client cannot apply
raises it while the token request is being built, before the endpoint is
contacted. Say so, parallel to the OAuthRegistrationError half of the
same sentence.

No-Verification-Needed: doc-only change

Author: maxisbey

docs/client/oauth-clients.md

@@ -131,7 +131,7 @@ There is one more no-human situation: the client belongs to an enterprise whose
 
 ## When it fails
 
-When the OAuth flow goes wrong, the provider raises an `OAuthFlowError` from `mcp.client.auth`. It has two subclasses. `OAuthRegistrationError` means registration did not yield a client you can use: the authorization server refused to register you, or it did register you but with credentials this flow cannot use (for instance an authentication method it does not implement). `OAuthTokenError` means the token endpoint said no. One `except OAuthFlowError:` covers discovery, registration, authorization, and exchange.
+When the OAuth flow goes wrong, the provider raises an `OAuthFlowError` from `mcp.client.auth`. It has two subclasses. `OAuthRegistrationError` means registration did not yield a client you can use: the authorization server refused to register you, or it did register you but with credentials this flow cannot use (for instance an authentication method it does not implement). `OAuthTokenError` means a token could not be obtained: the token endpoint said no, or a stored client record carries an authentication method this client cannot apply, which is reported while building the token request rather than sent. One `except OAuthFlowError:` covers discovery, registration, authorization, and exchange.
 
 Not everything is a flow error. The network can still fail; those are ordinary `httpx2` exceptions and pass through untouched.