Document and add warning that geo-position doesn't support selection yet

[kanitw]

(Might be work testing if something is supported, for example, I believe that single/multi selection that projects on non-position channel should still work.)

[jakevdp]

Here's a selection on a geo visualization that works correctly:

{
  "$schema": "https://vega.github.io/schema/vega-lite/v2.json",
  "config": {
    "view": {
      "height": 300,
      "width": 400
    }
  },
  "layer": [
    {
      "data": {
        "format": {
          "feature": "states",
          "type": "topojson"
        },
        "url": "https://vega.github.io/vega-datasets/data/us-10m.json"
      },
      "height": 400,
      "mark": {
        "fill": "lightgray",
        "stroke": "white",
        "type": "geoshape"
      },
      "projection": {
        "type": "albersUsa"
      },
      "title": "US State Capitols",
      "width": 700
    },
    {
      "data": {
        "url": "https://vega.github.io/vega-datasets/data/us-state-capitals.json"
      },
      "encoding": {
        "color": {
          "value": "black"
        },
        "size": {
          "condition": {
            "selection": {
              "not": "selector011"
            },
            "value": 30
          },
          "value": 100
        },
        "x": {
          "field": "lon",
          "type": "longitude"
        },
        "y": {
          "field": "lat",
          "type": "latitude"
        }
      },
      "mark": "point",
      "selection": {
        "selector011": {
          "fields": [
            "lat",
            "lon"
          ],
          "nearest": true,
          "on": "mouseover",
          "type": "single"
        }
      }
    },
    {
      "data": {
        "url": "https://vega.github.io/vega-datasets/data/us-state-capitals.json"
      },
      "encoding": {
        "opacity": {
          "condition": {
            "selection": {
              "not": "selector011"
            },
            "value": 0
          },
          "value": 1
        },
        "text": {
          "field": "city",
          "type": "nominal"
        },
        "x": {
          "field": "lon",
          "type": "longitude"
        },
        "y": {
          "field": "lat",
          "type": "latitude"
        }
      },
      "mark": {
        "align": "right",
        "dy": -5,
        "type": "text"
      }
    }
  ]
}

[kanitw]

That makes sense. I believe that the tricky part would be geo-position.
(Like what happen when we do interval selection over x and y.)

I'll close this since we should experiment more and we already have #3306.