Fixes #1873 : Seems to be working.

[Dunbaratu]

Test case examples:

// Should (if on kerbin near KSC) draw a white vector straight east
// at magnitude 174.94 m :
set vd1 to vecdraw(
  v(0,0,0),
  ship:geoposition:velocity:orbit,
  white,"velocity here",1,true).

// Should (if on kerbin near KSC) draw a nearly zero vector because
// surface-relative velocity of the ground right under me is negligable:
set vd1_s to vecdraw(
  v(0,0,0),
  ship:geoposition:velocity:surface,
  white,"srf velocity here",1,true).

// Should (if on kerbin near KSC) draw a surface vector like vd1 above,
// but longer in length because it's the rotational effect with a radius
// that's 200,000m bigger:
set vd2 to vecdraw(
  v(0,0,0),
  ship:geoposition:altitudevelocity(200000):orbit,
  green,"geoposition velocity higher up above me",1,true).

// Should (if on kerbin near KSC) draw a vector angled upward by
// 30 degreees because it's the velocity of the ground 30 longitude
// degrees west of the current position:
set vd3 to vecdraw(
  v(0,0,0),
  latlng(ship:latitude, ship:longitude-30),
  red,"velocity 30 degrees west of here",1,true).

// Should (if on kerbin near KSC) draw a vector angled upward by
// 30 degreees like above,  but this time it should be a much
// shorter vector because it's how fast the surface of the
// planet is moving much higher north, near the pole:
set vd4 to vecdraw(
  v(0,0,0),
  latlng(ship:latitude-80, ship:longitude-30),
  yellow,"velocity 30 deg west and 80 deg north of here",1,true).

Fixes #1873

[Dunbaratu]

I just realized the docs were missing from the commit, and added them. It should be ready now.