The original is explaining the negation, when the body would not be
included. Which would happen in the complement of the if expression,
ie. flipped by De Morgan's law's:

```
not (or .Values.anUnsetVariable (not .Values.aSetVariable))
==
and (not .Values.anUnsetVariable) .Values.aSetVariable
```

> unset variables evaluate to false

is equivalent to `not .Values.anUnsetVariable`.

> and

is equivalent to `and`.

> .Values.setVariable was negated with the not function

doesn't seem to match `.Values.aSetVariable`.
To me, that would be `not .Values.aSetVariable` instead.

Anyway, explaining the `if` expression as-is and not the negation is
clearer and parallels the first `if` operator.
Signed-off-by: Kevin Lau <kelau1993@gmail.com>