doc: clarify treatment of non-string base in URL()

PR-URL: #41685
Reviewed-By: Mohammed Keyvanzadeh <[email protected]>
Reviewed-By: Darshan Sen <[email protected]>

Author: Trott

doc/api/url.md

@@ -132,8 +132,8 @@ return `true`.
 * `input` {string} The absolute or relative input URL to parse. If `input`
   is relative, then `base` is required. If `input` is absolute, the `base`
   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.
+* `base` {string} The base URL to resolve against if the `input` is not
+  absolute. If `base` is not a string, it is [converted to a string][] first.
 
 Creates a new `URL` object by parsing the `input` relative to the `base`. If
 `base` is passed as a string, it will be parsed equivalent to `new URL(base)`.