Docs: Improve readability of the default arguments of `wp_list_categories()`.

Adds missing documentation for the `separator` argument introduced in [35140].

Props birgire for initial patch.
Fixes #34751.
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git-svn-id: https://core.svn.wordpress.org/trunk@36101 1a063a9b-81f0-0310-95a4-ce76da25c4cd