Clarify operator docs wording

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>

docs/chart_template_guide/functions_and_pipelines.md

@@ -159,7 +159,7 @@ Operators are implemented as functions that return a boolean value. To use `eq`,
 {{ end }}
 
 
-{{/* do not include the body of this if statement because unset variables evaluate to false and .Values.setVariable was negated with the not function. */}}
+{{/* include the body of this if statement when the variable .Values.anUnsetVariable is set or .values.aSetVariable is not set */}}
 {{ if or .Values.anUnsetVariable (not .Values.aSetVariable) }}
    {{ ... }}
 {{ end }}