minor improvements to turn_angle api docs

Author: karussell

docs/web/api-doc.md

@@ -95,7 +95,7 @@ paths[0].instructions[0].annotation_text      | [optional] A text describing the
 paths[0].instructions[0].annotation_importance| [optional] 0 stands for INFO, 1 for warning, 2 for costs, 3 for costs and warning
 paths[0].instructions[0].exit_number          | [optional] Only available for USE_ROUNDABOUT instructions. The count of exits at which the route leaves the roundabout.
 paths[0].instructions[0].exited               | [optional] Only available for USE_ROUNDABOUT instructions. True if the roundabout should be exited. False if a via point or end is placed in the roundabout, thus, the roundabout should not be exited due to this instruction.
-paths[0].instructions[0].turn_angle           | [optional] Only available for USE_ROUNDABOUT instructions. The radian of the route within the roundabout: 0<r<2*PI for clockwise and -2PI<r<0 for counterclockwise transit. Null if the direction of rotation is undefined.
+paths[0].instructions[0].turn_angle           | [optional] Only available for USE_ROUNDABOUT instructions. The radian of the route within the roundabout: `0 < r < 2*PI` for clockwise and `-2PI < r < 0` for counterclockwise transit. `NaN` if the direction of rotation is undefined.
 
 ```json
 {