Remove inaccurate comments (#23056)

These comments are inaccurate. Even if `convert_case` provided a way to
customize which boundaries were used (which is now does, it 0.7.1), they
would be removed from the string and replaced with the new boundary
character (`-`, `_`, ...), and we'd lose the ability to reconstruct the
text the way the author formatted it. This is not a hack, this is the
way we have to do it.

Release Notes:

- N/A