Add terms to wetland

[boothym]

I was retagging an old natural=marsh so searched for marsh in the presets, but didn't find anything. I've added the top five wetland=* values to help people find the wetland preset.

Maybe these could be made into proper presets in the future?

[bhousel]

Thanks @boothym! This is a good idea to help people find the wetland preset.

I don't think we need to make sub presets for all the different types, because there is already a wetland=* field (though I'm totally fine if somebody wants to do it, similar to how we have water=* sub presets: lake, pond, etc).