Add inline documentation for the 'show_admin_column' argument added to `register_taxonomy()` in 3.5.

Also adds a 'show_admin_column' value to the defaults array.

Props johnbillion.
Fixes #26707.


git-svn-id: https://develop.svn.wordpress.org/trunk@26912 602fd350-edb4-49c9-b593-d223f7449a82