Update docs about ID construction

[zbycz]

https://imposm.org/docs/imposm3/latest/mapping.html#id - it says that:

The ID of the OSM node, way or relation. Relation IDs are negated (-1234 for ID 1234) to prevent collisions with way IDs.

As said in https://github.com/omniscale/imposm3/issues/239 the logic is more like

• node = positive,
• way negative
• and relation negative < - 1e17.

[olt]

This only applies to use_single_id_space which itself is an undocumented feature.