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

[jo-asplin-met-no]

The documentation of the coords query parameter must 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

Affected endpoints:

• /position
• /area