document ScMarkArray a little

because it took a loooong time staring at the code to figure it out

Change-Id: Ia8f3b835cd7d18737e4b0ffe1aa67ad001822d82
Reviewed-on: https://gerrit.libreoffice.org/72395
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>
diff --git a/sc/inc/markarr.hxx b/sc/inc/markarr.hxx
index d30168d..54059fc 100644
--- a/sc/inc/markarr.hxx
+++ b/sc/inc/markarr.hxx
@@ -31,6 +31,11 @@ struct ScMarkEntry
    bool            bMarked;
};

/**
  This is a rather odd datastructure. We store alternating marked/not-marked entries,
  and for each entry the range is defined as :
      [previousEntry.nRow+1, currentEntry.nRow]
*/
class ScMarkArray
{
    SCSIZE                            nCount;