Update `bp_is_group_admin_screen()` to use `bp_is_group_admin_screen()`, and add phpdoc block.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8825 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
`bp_is_group_admin_screen()` returns bool, not string.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8826 cdf35c40-ae34-48e0-9cc9-0c9da1808c22