Document ents path / filename expectations (ZoneBuilder vs IW4x)

We need to actually document how ents overriding works. What is currently *non-obvious*:

* **ZoneBuilder** expects ents at
  ```
  /mapents/mp_mapname.ents
  ```

* **IW4x**, when overriding map ents, expects them at
  ```
  /maps/mp/mp_mapname.d3dbsp.ents
  ```

* Note the **different path and filename**. This is intentional and historical, but undocumented.

Explanation taken from @Rackover <3