[Tagging] Fire_hydrant: tagging namespace documentation

[Moritz]

> 
> In the discussion page [0] someone says that check_date=* is a
> synonymous of survey:date=* in common usage. Is this correct? Should
> we use another tag functional_check=* ?  But I don't like to introduce
> a new tag.
> 
+1 for not introducing a new tag.

But I think we need two different types of date tags.

First the survey date, aka a local mapper mapped the hydrant and saw it 
on the ground.
Second the check date, which should contain the date where the last 
functional check was done.

We should not complicate the functional checks. As first approach a 
general functional check is sufficient.
If we divide the functional check into flow, accessibility etc. we will 
have a never ending story with this proposal.

If e.g. it was a check for the flow, then a change in the flow_rate 
should indicate the check.

What further information do we gain if we separate different checks?

operational_status is not an official tag, but widely used [1].

So I would add it to the proposal to reflect different status of the 
hydrant.

[1]: https://taginfo.openstreetmap.org/keys/operational_status#values