FAQ: Document how to convert projects at different times.

In r2685, I documented how to do multiproject conversions the "new"
way, using an options file with multiple calls to
run_options.add_project().  But the "old way" (using intermediate
dumpfiles) still has value, because the user might want to migrate
different projects at different times.  So restore part of the old
text and add some new tips to reflect other improvements that have
been made in cvs2svn since then.

git-svn-id: http://cvs2svn.tigris.org/svn/cvs2svn/trunk@5449 be7e6eca-30d4-0310-a8e5-ac0d63af7087