Inline documentation for hooks in wp-comments-post.php.

props rzen.
see #25229.

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Inline documentation for the welcome_panel hook.

props natejacobs.
see #25229.

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Fixes for hook inline docs in wp-comments-post.php. see #25229.

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Hook docs for admin-footer.php.

props natejacobs.
see #25229.

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Document the trackback_post hook in wp-trackback.php.

props bananastalktome.
see #25229.

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Remove lingering instances of call time pass-by-reference, limited to instances of `callable` - use `$this` instead of `&$this`.

Props jdgrimes.
See #25160.


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When `meta_type` is passed with `orderby => meta_value`, `orderby` must also use `CAST()` to avoid scenarios like: `SELECT`ing by `UNSIGNED` and then ordering by `CHAR`. Adds unit test.

Fixes #21621.

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