Help: drop confusing reference to `if()` from `$<BOOL:>`

`if()` uses the internal `IsOn` call whild `$<BOOL:>` uses `IsOff`. For
historical reasons, these are not inverses of each other, so drop
mention of `if()` from the documentation of `$<BOOL:>`.
4 jobs for clarify-BOOL-genex-eval
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