[NFC] dev/core#1352 - Add comments why there's two custom blocks on new case form

[demeritcowboy]

Overview

Adds comments to new case template so someone doesn't get confused like I did.

I'll rant elsewhere about case custom fields, but because new case is the open case activity, it's possible to have custom field blocks that come from both case and activity. So this adds comments explaining it's not a duplicate.

[civibot]

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[seamuslee001]

Test fail unrelated