DOCS: state that initial_properties are shared between entity instances

doc/lua_api.md

@@ -8915,7 +8915,10 @@ Player properties need to be saved manually.
 Entity definition
 -----------------
 
-Used by `minetest.register_entity`.
+Used by `minetest.register_entity`.  
+The entity definition table becomes a metatable of a newly created per-entity
+luaentity table, meaning its fields (e.g. `initial_properties`) will be shared
+between all instances of an entity.
 
 ```lua
 {