doc: refine util.parseArgs default definition

The default value is legal by means other than defaulting.

PR-URL: #58958
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>

Author: Slayer95

doc/api/util.md

@@ -1955,10 +1955,12 @@ changes:
       times. If `true`, all values will be collected in an array. If
       `false`, values for the option are last-wins. **Default:** `false`.
     * `short` {string} A single character alias for the option.
-    * `default` {string | boolean | string\[] | boolean\[]} The default value to
-      be used if (and only 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.
+    * `default` {string | boolean | string\[] | boolean\[]} The value to assign to
+      the option if it does not appear in the arguments to be parsed. The value
+      must match the type specified by the `type` property. If `multiple` is
+      `true`, it must be an array. No default value is applied when the option
+      does appear in the arguments to be parsed, even if the provided value
+      is falsy.
   * `strict` {boolean} Should an error be thrown when unknown arguments
     are encountered, or when arguments are passed that do not match the
     `type` configured in `options`.