[Tagging] waterway=derelict_canal

[Mateusz Konieczny]

To people using this tag: please, update and clarify documentation of
this tag on OSM wiki. For example it is even unclear whatever it should
be expected that it can be applied to canals no longer filled with
water.

"used for transportation, waterpower, or irrigation" so filled by water
seems intended as implied - but canal that is drained and converted
into cycleway also fits "transportation".

On the other hand name (at least for me) implies canal where earthwork
is present but is no longer filled with water.

See http://wiki.openstreetmap.org/wiki/Tag:waterway%3Dderelict_canal
and https://github.com/gravitystorm/openstreetmap-carto/issues/1003