doc: "if and only if" should be "if" in util.parseArgs default value

[Slayer95]

The default value is legal by means other than defaulting.

[himself65]

upstream comment:

#54431 (comment)

[Slayer95]

Re: #54431, the critical part is "the option does not appear in the arguments to be parsed".

However, the way it's currently worded "if and only if", it literally means that the value passed as default behaves as a sentinel value: when it matches, the option was missing; otherwise, the option was in the arguments provided.

But that's not how it works. It's a one-side implication.

[aduh95]

Well it says "to be used", so I don't think it says the value could not be the default value if the option appears in the arguments.
Maybe we can try to rephrase it further to remove the ambiguity:

    * `default` {string | boolean | string\[] | boolean\[]} It must be of the
      same type as the `type` property. When `multiple` is `true`, it must be an
      array. It is ignored when when the option appears in the arguments to be
      parsed.

[shadowspawn]

@aduh95 That leaves out what default is actually used for. Adding your suggestion to the previous text:

`default` {string | boolean | string\[] | boolean\[]} The default value to
      be used if the option does not appear in the arguments to be parsed,
      and ignored if the option does appear. It must be of the same type as the
     `type` property. When `multiple` is `true`, it must be an array. 

[Slayer95]

`default` {string | boolean | string\[] | boolean\[]} The default value to
      be used if the option does not appear in the arguments to be parsed,
      and ignored if the option does appear. It must be of the same type as the
     `type` property. When `multiple` is `true`, it must be an array. 

This wording is also an "if and only if", just more verbose. This wording implies that if string option --foo has a default value of "bar", then --foo=bar is ignored.

I'd go with this:

* `default` {string | boolean | string\[] | boolean\[]} The value to be used if the
      option does not appear in the arguments to be parsed. It must be of the
      same type as the `type` property. When `multiple` is `true`, it must be an
      array. It is ignored when the option appears in the arguments to be
      parsed.

[shadowspawn]

(There is a "when when" in the last line.)