updateSummarySelected(IStructuredSelection selection) Retrieves statistical information (the total number of markers with each
severity type) for the markers contained in the selection passed in.
Retrieves statistical information (the total number of markers with each
severity type) for the markers contained in the selection passed in. This
information is then massaged into a string which may be displayed by the
caller.
Parameters: selection - a valid selection or null a message ready for display