fpspreadsheet: In TsWorksheetGrid, draw a borderline along frozen panes. Fix black background color appearing when re-saving an fpspreadsheet-created workbook in Excel. Some more extended comments for doc-o-matic.

git-svn-id: http://svn.code.sf.net/p/lazarus-ccr/svn@3224 8e941d3f-bd1b-0410-a28a-d453659cc2b4