api-doc: return status for notmuch_query_search_threads_st ...

[Gaute Hope]

Gaute Hope writes on February 7, 2016 11:06:
> Hi,
> 
> I am switching from the deprecated notmuch_query_search_threads to
> notmuch_query_search_threads_st, but the api-documentation does not
> specify what status values are returned and what they mean.
> 
> The same goes for notmuch_query_search_messages_st (...).

Indeed the typical usage section is also outdated (for both):

```
notmuch_query_t *query;
notmuch_messages_t *messages;
notmuch_message_t *message;

query = notmuch_query_create (database, query_string);

for (messages = notmuch_query_search_messages (query);
     notmuch_messages_valid (messages);
     notmuch_messages_move_to_next (messages))
{
    message = notmuch_messages_get (messages);
    ....
    notmuch_message_destroy (message);
}

notmuch_query_destroy (query);
```

-gaute