Add note about GetLegendGraphic and SLD_VERSION

[geographika]

Update the docs to match the implementation in mapwms.cpp.

if (nVersion >= OWS_1_3_0 && sld_version == NULL) {
  msSetErrorWithStatus(MS_WMSERR, MS_HTTP_400_BAD_REQUEST,
                       "Missing required parameter SLD_VERSION",
                       "GetLegendGraphic()");
  return msWMSException(map, nVersion, "MissingParameterValue",
                        wms_exception_format);
}
if (nVersion >= OWS_1_3_0 && strcasecmp(sld_version, "1.1.0") != 0) {
  msSetErrorWithStatus(MS_WMSERR, MS_HTTP_400_BAD_REQUEST,
                       "SLD_VERSION must be 1.1.0", "GetLegendGraphic()");
  return msWMSException(map, nVersion, "InvalidParameterValue",
                        wms_exception_format);
}

[geographika]

Although it does seem strange to have to add that parameter, when it can only ever be one value. GetLegendGraphic doesn't seem to be part of the WMS spec so I'm not sure why it couldn't be added in the code automatically if missing.