doc: doc and test URLSearchParams discrepancy

doc/api/url.md

@@ -472,9 +472,27 @@ and [`url.format()`][] methods would produce.
 * {URLSearchParams}
 
 Gets the [`URLSearchParams`][] object representing the query parameters of the
-URL. This property is read-only; to replace the entirety of query parameters of
-the URL, use the [`url.search`][] setter. See [`URLSearchParams`][]
-documentation for details.
+URL. This property is read-only but the `URLSearchParams` object it provides
+can be used to mutate the URL instance; to replace the entirety of query
+parameters of the URL, use the [`url.search`][] setter. See
+[`URLSearchParams`][] documentation for details.
+
+Care must be taken when using `.searchParams` to modify the `URL` because,
+per the WHATWG specification, the `URLSearchParams` object uses slightly
+different rules to determine which characters should be percent-encoded. For
+instance, the `URL` object will not percent encode the ASCII tilde (`~`)
+character, while `URLSearchParams` will always encode it:
+
+```js
+const myUrl = new URL('https://example.org/abc?foo=~bar');
+
+console.log(myUrl.search);  // prints ?foo=~bar
+
+// Modify the URL via searchParams...
+myUrl.searchParams.sort();
+
+console.log(myUrl.search);  // prints ?foo=%7Ebar
+```
 
 #### `url.username`
 

test/parallel/test-whatwg-url-custom-searchparams-stringifier.js

@@ -16,3 +16,12 @@ const URLSearchParams = require('url').URLSearchParams;
     message: 'Value of "this" must be of type URLSearchParams'
   });
 }
+
+// The URLSearchParams stringifier mutates the base URL using
+// different percent-encoding rules than the URL itself.
+{
+  const myUrl = new URL('https://example.org?foo=~bar');
+  assert.strictEqual(myUrl.search, '?foo=~bar');
+  myUrl.searchParams.sort();
+  assert.strictEqual(myUrl.search, '?foo=%7Ebar');
+}