Clarify how to specify ambient_music paths

docs/entityguide/entities/ambient_music.md

@@ -8,7 +8,7 @@ You can play and loop music, trigger it to fade out or stop it entirely.
 
 ## Keyvalues
 
-* **filename** - Relative name of the file to play. Only applies to the `Play` and `Loop` commands
+* **filename** - Relative name of the file to play, starting in the root directory. For example: `media/Half-Life01.mp3`. Only applies to the `Play` and `Loop` commands
 * **command** - The command to issue. Any of `Play, Loop, Fadeout, Stop`
 * **target_selector** - How to select players to affect. Any of `AllPlayers, Activator, Radius`
 * **radius** - If `target_selector` is `Radius`, this is how close the player has to be to be affected by this entity