Add note to FAQ about recycled entity ids

docs/FAQ.md

@@ -46,9 +46,23 @@ This is likely because you're creating a query in a loop. Queries should be crea
 ## Can Flecs be compiled to web assembly?
 Yes it can! See the [quickstart manual](Quickstart.md) for more information.
 
-## Why am I getting an “use of undeclared identifier 'FLECS__E …’” compiler error?
+## Why am I getting an “use of undeclared identifier 'FLECS_ID …’” compiler error?
 This happens in C if the variable that holds the component id can't be found. This example shows how to fix it: https://github.com/SanderMertens/flecs/tree/master/examples/c/entities/fwd_declare_component
 
+## Why are my entity ids so large?
+When you inspect the integer value of an entity you may see that this value is very large, usually around 4 billion. This is not a bug, and instead means that the entity id has been recycled. This example shows when recycling happens:
+
+```cpp
+flecs::entity e1 = world.entity(); // small id
+e1.destruct(); // id is made available for recycling
+
+flecs::entity e2 = world.entity(); // recycles e1
+e1.is_alive(); // false
+e2.is_alive(); // true
+
+std::cout << e2.id(); // large id, upper 32 bits contains liveliness count
+```
+
 ## What is the difference between add & set? Why do both exist?
 An `add` just adds a component without assigning a value to it. A `set` assigns a value to the component. Both operations ensure that the entity will have the component afterwards.