Update custom unavailable slot comment

Co-authored-by: chris <github@onechris.mozmail.com>

core/src/main/java/org/geysermc/geyser/configuration/AdvancedConfig.java

@@ -62,7 +62,9 @@ public interface AdvancedConfig {
 
     @Comment("""
             Which item to use to mark unavailable slots in a Bedrock player inventory. Examples of this are the 2x2 crafting grid while in creative,
-            or custom inventory menus with sizes different from the usual 3x9. A barrier block is the default item.""")
+            or custom inventory menus with sizes different from the usual 3x9. A barrier block is the default item.
+            This config option can be set to any Bedrock item identifier. If you want to set this to a custom item, make sure that you specify the item in the following format: "geyser_custom:<mapping-name>"
+            """)
     @DefaultString("minecraft:barrier")
     String unusableSpaceBlock();