[Tagging] Clarification of fire_hydrant:diameter

[Marc Gemis]

In Belgium it is the inner diameter of the water main. Lowest value
I've seen is 50, highest 400 or so.
It's nicely explained on
http://brandweerbrasschaat.be/site/content/hydranten (in Dutch) with
pictures of the signs. The small ref number in the upper right corner
is not always there.
If you ever need pictures of those signs, please contact me, I have
plenty of them, but I have to look them up.

m.

p.s. the current JOSM preset & associated validation gives warnings
for 90, which is not uncommon here.