[API] Complete documentation of the 'coords' query parameter

[jo-asplin-met-no]

The documentation of the coords query parameter must ...

1. state explicitly the format of the WKT expression so that the API user has a chance of understanding what the expression means:

• the fact that the longitude component comes before the latitude component in each coordinate
• the fact that the first and last coordinate should be identical

2. make it clear that each line in the polygon follows the great circle arc between the two points (particularly useful to be aware of for long edges that go mostly from west to east)

Affected endpoints:

• /position
• /area