Update api-doc.md

Author: karussell

docs/web/api-doc.md

@@ -46,11 +46,11 @@ ch.disable       | `false`    | Use this parameter in combination with one or mo
 weighting        | `fastest`  | Which kind of 'best' route calculation you need. Other option is `shortest` (e.g. for foot and bike) and `curvature` (only for motorcycle)
 edge_traversal   |`false`     | Use `true` if you want to consider turn restrictions for bike and motor vehicles. Keep in mind that the response time is roughly 2 times slower.
 algorithm        |`dijkstrabi`| The algorithm to calculate the route. Other options are `dijkstra`, `astar`, `astarbi`, `alternative_route` and `round_trip`
-heading          | NaN        | Favour a heading direction for a certain point. Specify either one heading for the start point or as many as there are points. In this case headings are associated by their order to the specific points. Headings are given as north based clockwise angle between 0 and 360 degree.
+heading          | NaN        | Favour a heading direction for a certain point. Specify either one heading for the start point or as many as there are points. In this case headings are associated by their order to the specific points. Headings are given as north based clockwise angle between 0 and 360 degree. This parameter also influences the tour generated with `algorithm=round_trip` and force the initial direction.
 heading_penalty  | 120        | Penalty for omitting a specified heading. The penalty corresponds to the accepted time delay in seconds in comparison to the route without a heading.
 pass_through     | `false`    | If `true` u-turns are avoided at via-points with regard to the `heading_penalty`.
 round_trip.distance                 | 10000 | If `algorithm=round_trip` this parameter configures approximative length of the resulting round trip
-round_trip.seed                     | 0     | If `algorithm=round_trip` this parameter introduces randomness if e.g. the first try wasn't good
+round_trip.seed                     | 0     | If `algorithm=round_trip` this parameter introduces randomness if e.g. the first try wasn't good.
 alternative_route.max_paths         | 2     | If `algorithm=alternative_route` this parameter sets the number of maximum paths which should be calculated. Increasing can lead to worse alternatives.
 alternative_route.max_weight_factor | 1.4   | If `algorithm=alternative_route` this parameter sets the factor by which the alternatives routes can be longer than the optimal route. Increasing can lead to worse alternatives.
 alternative_route.max_share_factor  | 0.6   | If `algorithm=alternative_route` this parameter specifies how much alternatives routes can have maximum in common with the optimal route. Increasing can lead to worse alternatives.