Clarify is_polygon_clockwise() coordinate system

[lawnjelly]

The function assumes a normal Cartesian coordinate system.

Fixes #49716

Notes

• Alternative would be to add a parameter to do the flip as in Add handedness enum for is_polygon_clockwise #89635, but that may be controversial, and is taking a while to be decided on, so this doc change will do the job either as the final solution, or until we decide on another solution.

[AThousandShips]

Will likely be needed for older versions regardless of adding a handedness option down the line, unless it can be considered safe for cherry picking (and likely won't get into 4.3 anyway)

[akien-mga]

Thanks!