clarify the comment about libssh2_sftp_write's return type

lib/ssh.c

@@ -1018,7 +1018,8 @@ CURLcode Curl_sftp_done(struct connectdata *conn, CURLcode status,
 ssize_t Curl_sftp_send(struct connectdata *conn, int sockindex,
                        void *mem, size_t len)
 {
-  ssize_t nwrite;   /* libssh2_sftp_write() returns size_t !*/
+  ssize_t nwrite;   /* libssh2_sftp_write() used to return size_t in 0.14
+                       but is changed to ssize_t in 0.15! */
 
 #ifdef LIBSSH2SFTP_EAGAIN
   /* we prefer the non-blocking API but that didn't exist previously */