Taxonomies: Improve description of default category behaviour.

Add some extra information to the Categories list screen, explaining why the default category doesn't have a Delete button.

Props Codestor.
Fixes #44961.


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git-svn-id: https://core.svn.wordpress.org/trunk@43495 1a063a9b-81f0-0310-95a4-ce76da25c4cd