fpspreadsheet: Completing chm documentation of the fpspreadsheet unit (except for the CustomReader/Writer which I would like to skip but don't know how to do this with doc-o-matic).

git-svn-id: http://svn.code.sf.net/p/lazarus-ccr/svn@3211 8e941d3f-bd1b-0410-a28a-d453659cc2b4