tdf#165180: improve documentation of lowestEdited

Office 2013 with SP1 is 6, with a rupBuild of 14420
Office 2016 is also 6, with a rupBuild of 14420 also (surprisingly).

Change-Id: Ib9b5906e97d8e39838ecda04bdc23fdf47d197a8
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/195601
Tested-by: Jenkins
Reviewed-by: Justin Luth <jluth@mail.com>

oox/source/core/filterdetect.cxx

@@ -179,7 +179,7 @@ void FilterDetectDocHandler::parseWorkbook(const AttributeList& rAttribs)
 
     // tdf#165180 Remember filter when opening file as 'Office Open XML Spreadsheet'
     // (fileVersion can only exist once, and lowestEdited can only be defined once - else corrupt)
-    // lowestEdited: 4 is 2007, 5 is 2010, 6 is 201?, 7 is 201?-2024
+    // lowestEdited: 4 is 2007, 5 is 2010, 6 is 2013 and 2016, 7 is 2019-2024
     const sal_Int32 nDefaultValue = rAttribs.getInteger(XML_lastEdited, 99);
     if (rAttribs.getInteger(XML_lowestEdited, nDefaultValue) > 4)
         maOOXMLVariant = OOXMLVariant::ISO_Transitional; // Excel 2010+

sc/inc/scextopt.hxx

@@ -30,7 +30,10 @@ struct ScExtDocSettings
     double              mfTabBarWidth;      ///< Width of the tabbar, relative to frame window width (0.0 ... 1.0).
     sal_uInt32          mnLinkCnt;          ///< Recursive counter for loading external documents.
     SCTAB               mnDisplTab;         ///< Index of displayed sheet.
-    std::optional<sal_Int16> moLowestEdited; ///< Oldest Excel version that edited this document.
+    /** moLowestEdited:  oldest Excel version that edited this .XLSX - limit feature set to that.
+     *  4 is 2007, 5 is 2010, 6 is 2013 and 2016, 7 is 2019-2024+
+     */
+    std::optional<sal_Int16> moLowestEdited;
 
     explicit            ScExtDocSettings();
 };