Here's a good reference about the pcb silk screen layer. I've seen many posts about assembly errors because of inadequate silk screen labeling, or had difficulties trying to follow a board layout because parts aren't clearly labeled. This reference may be useful.

Essential PCB Silkscreen Guidelines for Layout

 

It is conventional to mark the cathode of a rectifying diode with the + sign, and it is also conventional to mark the anode of an LED with the + sign.
Use 'a' and 'k' or draw the diode symbol. If you use + signs it will inevitably be assembled wrongly.