doc: discuss special protocol handling

doc/api/url.md

@@ -389,6 +389,46 @@ console.log(myURL.href);
 
 Invalid URL protocol values assigned to the `protocol` property are ignored.
 
+##### Special Schemes
+
+The [WHATWG URL Standard][] considers a handful of URL protocol schemes to be
+_special_ in terms of how they are parsed and serialized. When a URL is
+parsed using one of these special protocols, the `url.protocol` property
+may be changed to another special protocol but cannot be changed to a
+non-special protocol, and vice versa.
+
+For instance, changing from `http` to `https` works:
+
+```js
+const u = new URL('http://example.org');
+u.protocol = 'https';
+console.log(u.href);
+// https://example.org
+```
+
+However, changing from `http` to a hypothetical `fish` protocol does not
+because the new protocol is not special.
+
+```js
+const u = new URL('http://example.org');
+u.protocol = 'fish';
+console.log(u.href);
+// http://example.org
+```
+
+Likewise, changing from a non-special protocol to a special protocol is also
+not permitted:
+
+```js
+const u = new URL('fish://example.org');
+u.protocol = 'http';
+console.log(u.href);
+// fish://example.org
+```
+
+The protocol schemes considered to be special by the WHATWG URL Standard
+include: `ftp`, `file`, `gopher`, `http`, `https`, `ws`, and `wss`.
+
 #### url.search
 
 * {string}