Document clipping strategy, and clip_extent

[martinfrances107]

This is a follow up to the russia rendering issue

The documented description of the difference between antemeridian clipping and circle clipping is poor and incorrect.

here is a good article I should reference

https://www.jasondavies.com/maps/clip/

Separately the ability to setting the "Clip Extent" is not mentioned

( A map polygon part in a and part outside this bound will be "clipped", that is reshaped so as it fit inside )