Game_api.txt: Add position table reference

game_api.txt

@@ -12,6 +12,7 @@ Please note:
 
  * [XYZ] refers to a section the Minetest API
  * [#ABC] refers to a section in this document
+ * [pos] refers to a position table `{x = -5, y = 0, z = 200}`
 
 Bucket API
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