Document field_door code

ld_script.txt

@@ -94,7 +94,6 @@ SECTIONS {
         src/metatile_behavior.o(.text);
         asm/field_camera.o(.text);
         src/field_door.o(.text);
-        asm/field_door.o(.text);
         asm/field_player_avatar.o(.text);
         asm/field_map_obj.o(.text);
         asm/field_ground_effect.o(.text);