Update the database layout documentation

Update the database layout documentation

The place table holds a single row per OSM object now, with a categories
column that carries all of its main tags. Document that, the categories
column on placex and the tables that were added since the page was last
touched: place_entrance, placex_entrance, place_associated_street and
location_area_country. The place_classtype tables are gone, so drop them
from the list of auxiliary tables.

Also bring the diagrams back in line with the schema. search_name lost
its search_rank column a while ago and location_postcodes gained is_area,
while its centroid was missing from the diagram because of a typo.