lwip_netconn_do_writemore() cleanups

This commit makes a couple of cleanups discussed in patch #8882:
  1) msg.w.offset should not be set to 0 in the error case.  It is
     only valid when err == ERR_OK
  2) Remove out-of-date comment which indicated the entire write had
     completed (not true for non-blocking write)

This also updates the documentation on offset to include that offset
is only valid when err == ERR_OK

src/include/lwip/priv/api_msg.h

@@ -106,7 +106,7 @@ struct api_msg {
       const void *dataptr;
       /** total length of dataptr */
       size_t len;
-      /** offset into dataptr/output of bytes written */
+      /** offset into dataptr/output of bytes written when err == ERR_OK */
       size_t offset;
       u8_t apiflags;
 #if LWIP_SO_SNDTIMEO