unitable example: explain more about how geometries are added

Author: lonvia

flex-config/unitable.lua

@@ -5,6 +5,9 @@
 -- inspect = require('inspect')
 
 -- We define a single table that can take any OSM object and any geometry.
+-- OSM nodes are converted to Points, ways to LineStrings and relations
+-- to GeometryCollections. If an object would create an invalid geometry
+-- it is still added to the table with a NULL geometry.
 -- XXX expire will currently not work on these tables.
 local dtable = osm2pgsql.define_table{
     name = "data",