Document only-merge-items-horizontally (#461)

Author: lynxplay

Diff for: config-specs/paper/paper-world-defaults.yml

@@ -250,6 +250,11 @@ entities:
     nerf-pigmen-from-nether-portals:
       default: "false"
       description: "Removes AI from pigmen spawned via nether portals "
+    only-merge-items-horizontally:
+      default: "false"
+      description: >-
+        Prevents merging items that are not on the same y level, preventing potential
+        visual artifacts.
     parrots-are-unaffected-by-player-movement:
       default: "false"
       description: >-

Diff for: config-specs/paper/vanilla/paper-world-defaults.yml

@@ -12,6 +12,8 @@ collisions:
       The actual default value would be infinity. 2147483647 is the closest to infinity.
 entities:
   behavior:
+    only-merge-items-horizontally:
+      default: "true"
     phantoms-do-not-spawn-on-creative-players:
       default: "false"
     phantoms-only-attack-insomniacs: