document player.get_dir function (#695)

* update doc/*/scripting/builtins/libplayer.md

doc/en/scripting/builtins/libplayer.md

@@ -48,6 +48,12 @@ player.set_rot(playerid: int, x: number, y: number, z: number)
 
 Set camera rotation (degrees)
 
+```lua
+player.get_dir(playerid: int) -> vec3
+```
+
+Returns the player look direction vector
+
 ```lua
 player.get_inventory(playerid: int) -> int, int
 ```