doc: add url.resolve() usage examples

Fixes #4913.

doc/api/url.markdown

@@ -96,4 +96,8 @@ Take a parsed URL object, and return a formatted URL string.
 ## url.resolve(from, to)
 
 Take a base URL, and a href URL, and resolve them as a browser would for
-an anchor tag.
+an anchor tag.  Examples:
+
+    url.resolve('/one/two/three', 'four')         // '/one/two/four'
+    url.resolve('http://example.com/', '/one')    // 'http://example.com/one'
+    url.resolve('http://example.com/one', '/two') // 'http://example.com/two'