Improve inline documentation for `add_post_type_support()`.

Props philiparthurmoore.
Fixes #26683.

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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.

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Inline documentation for hooks in wp-admin/network/admin.php.

Props Hanni.
Fixes #26216.

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Inline documentation for hooks in wp-admin/network/upgrade.php.

Props miyauchi for the initial patch.
Fixes #25881.

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