Fix OAuth validator docs for error_detail on internal errors

Hi,

While testing “[d438a3659] oauth: Let validators provide failure DETAILs”, I noticed a tiny doc issue.

With this feature, when a validator returns false, result->error_detail can carry an error message. However, it seems that the previous paragraph was not updated:
```
   <para>
    A validator may return <literal>false</literal> to signal an internal error,
    in which case any result parameters are ignored and the connection fails.

```

“Any result parameters are ignored” is no longer accurate; it should be something like “any result parameters except result->error_detail are ignored”. This patch just makes that tiny doc fix.

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/