Add configuration option for ghost reset distance. Improve documentation.

ghost.h

@@ -3,6 +3,8 @@
 
 #include <glm/glm.hpp>
 
+// Ghost encapsulates AI state for a single floating enemy constrained to an
+// axis-aligned bounding box inside the maze.
 class Ghost {
     public:
         Ghost(float xmin, float xmax, float zmin, float zmax);
@@ -17,14 +19,14 @@ class Ghost {
         bool first_frame;
         glm::vec3 moved;
         glm::vec3 pos;
-        float yawr;
+        float yawr; // facing direction used while wandering
         float xmin;
         float xmax;
         float zmin;
         float zmax;
         float y_offset;
         float prev_move_time;
-        float direction_persist;
+        float direction_persist; // blends rotation over time so ghosts don't jitter
 };
 
 float rand_float(float rmin, float rmax);