tweak documentation and add new screenshot

git-svn-id: https://plugins.svn.wordpress.org/bepro-listings/trunk@816428 b8457f37-d9ea-0310-8a92-e5e31aec5664
add screenshot-7.jpg

git-svn-id: https://plugins.svn.wordpress.org/bepro-listings/trunk@816430 b8457f37-d9ea-0310-8a92-e5e31aec5664