doc: clarify treatment of non-string argument to new URL()

Closes: nodejs#41653

doc/api/url.md

@@ -129,9 +129,9 @@ return `true`.
 
 #### `new URL(input[, base])`
 
-* `input` {string} The absolute or relative input URL to parse. If `input`
+* `input` {any} The absolute or relative input URL to parse. If `input`
   is relative, then `base` is required. If `input` is absolute, the `base`
-  is ignored.
+  is ignored. If `input` is not a string, it is [converted to a string][] first.
 * `base` {string|URL} The base URL to resolve against if the `input` is not
   absolute.
 
@@ -1681,6 +1681,7 @@ console.log(myURL.origin);
 [Punycode]: https://tools.ietf.org/html/rfc5891#section-4.4
 [WHATWG URL]: #the-whatwg-url-api
 [WHATWG URL Standard]: https://url.spec.whatwg.org/
+[converted to a string]: https://tc39.es/ecma262/#sec-tostring
 [`Error`]: errors.md#class-error
 [`JSON.stringify()`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/JSON/stringify
 [`Map`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map