Extend documentation center line.

Add infos for center line

Author: carsten-kuebler

osi_lane.proto

@@ -56,9 +56,12 @@ message Lane
         //
         // This is the line, that a typical vehicle is supposed to follow. Thus, if
         // the road narrows (e.g. at a construction site) this line is supposed to
-        // describe the trajectory that the vehicle should follow. The intended
-        // direction of travel on the lane is given by the direction defined by the
-        // sequence of points forming the center line.
+        // describe the trajectory that the vehicle should follow. 
+        // Depending on the size of the vehicle, the vehicle may not be able to 
+        // orient itself directly on the center line due to its dimensions, but 
+        // may have to deviate from it in order to avoid e.g. a collision.
+        // The intended direction of travel on the lane is given by the
+        // direction defined by the sequence of points forming the center line.
         //
         // \image html highway_exit.png "Center lines as yellow dotted lines."
         //
@@ -72,6 +75,9 @@ message Lane
         //
         // \note The \c #centerline is defined only for \c #type =
         // \c #TYPE_DRIVING and one \c #lane_pairing pair.
+        // \note Intersections and non-driving lanes do not have a center line. 
+        // A vehicle must calculate this individually and depending on the 
+        // situation.
         //
         repeated Vector3d centerline = 3;