docs: PROTOCOL.md: Mention that the EDID blob pointer can be NULL

PROTOCOL.md

@@ -505,6 +505,8 @@ struct limine_framebuffer {
 `modes` is an array of `mode_count` pointers to `struct limine_video_mode` describing the
 available video modes for the given framebuffer.
 
+`edid` points to the screen's EDID blob, if available, else NULL.
+
 ```c
 struct limine_video_mode {
     uint64_t pitch;