curl_easy_setopt.3: prefer XFERINFOFUNCTION to PROGRESSFUNCTION

docs/libcurl/curl_easy_setopt.3

@@ -347,6 +347,8 @@ Pass a pointer to a function that matches the following prototype:
 \fBint function(void *clientp, double dltotal, double dlnow, double ultotal,
 double ulnow);\fP
 
+We encourage users to use \fICURLOPT_XFERINFOFUNCTION\fP instead, if you can.
+
 This function gets called by libcurl instead of its internal equivalent with a
 frequent interval. While data is being transferred it will be called very
 frequently, and during slow periods like when nothing is being transferred it