[mkgmap-dev] Documentation improvements for sea processing,

Hi Gerd, Please find attached a patch to improve the documentation of sea processing plus minor changes to --add-pois-to-lines. I note that extend-sea-sectors says "Adds a point so coastline reaches the nearest tile boundary. This implies no-sea-sectors". But no-sea-sectors disables the generation of sea sectors when the coastline fails to reach the tile's boundary, so I would have thought that extend-sea-sectors is an alternative action and would imply that no-sea-sectors is off. Cheers, Mike