[Tagging] Place=archipelago wiki page update

[Joseph Eisenberg]

I've just updated the wiki page for the in-use tag "place=archipelago".

The main changes are:
- Described an archipelago as a named chain, cluster or group of
closely related islands
- warning against creating giant multipolygons by mapping the whole
Philippines as a single multipolygon relation
- warning that "all the islands in this sea" is not a useful
archipelago; eg Macaronesia (=Canary Islands, Azores, Cape Verde
Islands...), "all the islands in the Aegean"
- Added info about benefits of using multipolygon relations vs cluster
relations (the later is not supported by most map users)
- mentioned that some are mapped as nodes (although this is not preferred)

The page already suggested that archipelagos should be mapped as a
relation that includes all of the coastlines of the islands as members
of the relation.

I believe with 740 uses this tag is clearly "in use", but not "de facto".

Any comments or suggestions for improvements?

- Joseph

(Also see Openstreetmap-Carto Issue #3394 about displaying the names
of archipelagos
https://github.com/gravitystorm/openstreetmap-carto/issues/3394)