Update documentation for is_land and is_territorial

[pslh]

Following discussions around and acceptance of OvertureMaps/schema#546 the documentation should be updated to better describe the intended use and behaviour of the is_land and is_territorial fields

There was a fair amount of discussion in the wg-schema Slack channel (Jun 11th) some of which included some descriptions and examples that could be useful for the documentation. In particular:

• landlocked countries: is_land=true is_territorial=true
• land clipped countries with water: is_land=true is_territorial=false
• land including water: is_land=false is_territorial=true

[pslh]

Tagging Stephen Epps (@stepps00)