Docs: Add a missing changelog entry for the point where the `$tagnames`...

Docs: Add a missing changelog entry for the point where the `$tagnames` parameter was added to `get_shortcode_regex()`.

Props keesiemeijer.
Fixes #38914.

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git-svn-id: https://core.svn.wordpress.org/trunk@39291 1a063a9b-81f0-0310-95a4-ce76da25c4cd