Polygonizer extractOnlyPolygonal documentation

[mukoki]

I found that extractOnlyPolygonal option is quite difficult to understand and would desserve more documentation.

• use case where I found it useful : building polygons from their boundaries (avoids to build polygons in original holes)
• situation where it makes no sense : on a linework representing limits of a coverage, the option build some faces and omit others with an unpredictable pattern (keep some adjacent faces and omit others). Worse : running several times the algo produces different patterns.