[matkoniecz]

> how to create a polygonal feature more complex than a single building footprint

In ID (default editor) you can mark area and area inside or select two disjointed areas and press right click on the and select "Merge". Or press "c" while selecting areas for combining.

In JOSM there is equivalent "create multipolygon" (or "update multipolygon")

https://wiki.openstreetmap.org/wiki/Relation:multipolygon#Ho...

> parks that shared a boundary with a road

FYI, that is because highway=* road line represents centerline of carriageway - and unless park somehow ends in the middle of road and includes half of its surface it will be not correct.

It also makes future editing quite nasty.