Clarify wikidata fountain terminologies

[mmmatthew]

This documents how terms should be used when documenting fountains in wikidata.

• Well and Spring have a direct connection to the groundwater
• Fountain = Springbrunnen is decorative and ejects water
• Drinking fountain gives potable water
• Pond is a body of water in the ground

These terms should be used carefully. Sometimes multiple terms will apply to a single fountain.

[ralfhauser]

as for https://www.wikidata.org/wiki/Q63715939 , I think we should only go for instances of man-made ponds (Q64129187)

and we might even add https://www.wikidata.org/wiki/Q2520975 (decoration) do it