Convert argument array documentation to the hash-notation style for `WP_Admin_Bar::add_node()` and `WP_Admin_Bar::add_group()`.

Fixes #27258.

Built from https://develop.svn.wordpress.org/trunk@27370


git-svn-id: http://core.svn.wordpress.org/trunk@27220 1a063a9b-81f0-0310-95a4-ce76da25c4cd