By default, the method computes a closed-loop length.
Being this public API, the only thing that can be done is to properly
document the behavior.

[ChangeLog][QtPositioning] Documented QGeoPath::length behavior properly.

Change-Id: Ie48a51ab0d11ec68851c2ea26b8c6f6894be8b61
Fixes: QTBUG-71358
Reviewed-by: Alex Blasche <alexander.blasche@qt.io>