CURLMOPT_PUSHFUNCTION.3: *_byname() returns only the first header

... if there are more than one using the same name

docs/libcurl/opts/CURLMOPT_PUSHFUNCTION.3

@@ -77,7 +77,8 @@ Returns the header at index 'num' (or NULL). The returned pointer points to a
 Returns the value for the given header name (or NULL). This is a shortcut so
 that the application doesn't have to loop through all headers to find the one
 it is interested in. The data pointed will be freed when this callback
-returns.
+returns. If more than one header field use the same name, this returns only
+the first one.
 .SH CALLBACK RETURN VALUE
 .IP "CURL_PUSH_OK (0)"
 The application has accepted the stream and it can now start receiving data,