document pop filters (Thorsten Zachmann); fix some fixme's
document pop filters (Thorsten Zachmann); fix some fixme's

svn path=/trunk/kdenetwork/doc/kmail/; revision=135623

lefou/kdepim-noakonadi,lefou/kdepim-noakonadi,lefou/kdepim-noakonadi,lefou/kdepim-noakonadi,lefou/kdepim-noakonadi,lefou/kdepim-noakonadi