REST API: Set the comment `type` to a readonly property in the schema.

Document the type property as `readonly` and remove the default value. After #38820 it is no longer possible to set the type property on a comment to anything a custom type.

Props jnylen0, rachelbaker.
Fixes #38886.
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git-svn-id: https://core.svn.wordpress.org/trunk@39277 1a063a9b-81f0-0310-95a4-ce76da25c4cd