Document (improve) custom placetypes

[thisisaaronland]

At a minimum provide an example. Silencing the placetype=alt warnings (for alternate geometries) would be a good example.

[straup]

Related, when working with not-WOF GeoJSON files:

2020/12/19 16:02:42 Unable to parse placetype (multipolygon) for ID 2344916, because 'Invalid placetype' - skipping placetype filters
2020/12/19 16:02:42 Unable to parse alternate geometry () for ID 2344916, because 'Unable to parse WOF ID' - skipping alternate geometry filters
2020/12/19 16:03:00 Unable to parse placetype (multipolygon) for ID 2344916, because 'Invalid placetype' - skipping placetype filters
2020/12/19 16:03:00 Unable to parse alternate geometry () for ID 2344916, because 'Unable to parse WOF ID' - skipping alternate geometry filters
2020/12/19 16:03:04 Unable to parse placetype (multipolygon) for ID 2344916, because 'Invalid placetype' - skipping placetype filters