Docs: Update the summary for the `comment_form_fields` hook docs to explicitly...

Docs: Update the summary for the `comment_form_fields` hook docs to explicitly mention inclusion of the textarea element as a filterable element.

Previously [35723]

Props bappidgreat.
Fixes #34973.

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git-svn-id: https://core.svn.wordpress.org/trunk@35841 1a063a9b-81f0-0310-95a4-ce76da25c4cd