Updated SLD cookbook 3-color gradient to match current behavior

Updated rasters.rst to match actual geoserver behaviour (Tested in GeoServer 2.6) - Values above the upper bound are in fact rendered when using default ColorMap type (If type="intervals" is used instead of the default, values above the upper bound are not rendered).

Updated raster_threecolorgradient.png to demonstrate this behaviour.

Small fix to the title of point_simplepoint.sld (Previously had the same title as point_simplepointwithstroke.sld)