[Tagging] Explain sport=multi

[Tobias Knerr]

Erik Johansson wrote:
> sport=multi is very well used but have no description in the wiki. Is
> there anyone that uses this tag?
> 
> For some reason I get the feeling this is at least when I see it used
> as a shorthand for multiple values on a sport key

I'd use sport=multi on a typical gym that can be used for essentially
every kind of (indoor) sport.

For example, the current usage of the gym near my home would be
something like:

sport=basketball;handball;badminton;gymnastics;fencing;hockey;volleyball;soccer;climbing;aikido;boxing;juggling;frisbee;...

And the exact list of sports would change every few months. So
sport=multi seems like the saner option.