fpspreadsheet: Rename TsWorksheetGrid method NewWorksheet to NewWorkbook (because that's what it does). Add doc-o-matic comments to fpspreadsheetgrid.pas.

git-svn-id: http://svn.code.sf.net/p/lazarus-ccr/svn@3229 8e941d3f-bd1b-0410-a28a-d453659cc2b4