Update Network Dashboard help text. props DrewAPicture, raggedrobins. see #23163.

git-svn-id: http://core.svn.wordpress.org/trunk@23657 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Replace deprecated WP_Screen::is_network property with WP_Screen::in_admin( 'network' ). props bpetty. fixes #23215.

git-svn-id: http://core.svn.wordpress.org/trunk@23658 1a063a9b-81f0-0310-95a4-ce76da25c4cd