Add inline documentation for the 'show_admin_column' argument added to...

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.

Built from https://develop.svn.wordpress.org/trunk@26912


git-svn-id: https://core.svn.wordpress.org/trunk@26793 1a063a9b-81f0-0310-95a4-ce76da25c4cd