Improved documentation of ImpulseManeuver class.

Fixes #838

Thank you to Kendra Hale.

src/main/java/org/orekit/forces/maneuvers/ImpulseManeuver.java

@@ -45,12 +45,19 @@ import org.orekit.utils.PVCoordinates;
  * can also be a more elaborate {@link
  * org.orekit.propagation.events.ApsideDetector apside event} for apogee
  * maneuvers for example.</p>
- * <p>The maneuver is defined by a single velocity increment in satellite
- * frame. The current attitude of the spacecraft, defined by the current
- * spacecraft state, will be used to compute the velocity direction in
- * inertial frame. A typical case for tangential maneuvers is to use a
- * {@link org.orekit.attitudes.LofOffset LOF aligned} attitude provider for state propagation and a
- * velocity increment along the +X satellite axis.</p>
+ * <p>The maneuver is defined by a single velocity increment.
+ * If no AttitudeProvider is given, the current attitude of the spacecraft,
+ * defined by the current spacecraft state, will be used as the
+ * {@link AttitudeProvider} so the velocity increment should be given in
+ * the same pseudoinertial frame as the {@link SpacecraftState} used to
+ * construct the propagator that will handle the maneuver.
+ * If an AttitudeProvider is given, the velocity increment given should be
+ * defined appropriately in consideration of that provider. So, a typical
+ * case for tangential maneuvers is to provide a {@link org.orekit.attitudes.LofOffset LOF aligned}
+ * attitude provider along with a velocity increment defined in accordance with
+ * that LOF aligned attitude provider; e.g. if the LOF aligned attitude provider
+ * was constructed using LOFType.VNC the velocity increment should be
+ * provided in VNC coordinates.</p>
  * <p>Beware that the triggering event detector must behave properly both
  * before and after maneuver. If for example a node detector is used to trigger
  * an inclination maneuver and the maneuver change the orbit to an equatorial one,