Issue #25570: Add example of customizing User-Agent via add_header()

Doc/library/urllib.request.rst

@@ -189,7 +189,7 @@ The following classes are provided:
 
    *headers* should be a dictionary, and will be treated as if
    :meth:`add_header` was called with each key and value as arguments.
-   This is often used to "spoof" the ``User-Agent`` header, which is
+   This is often used to "spoof" the ``User-Agent`` header value, which is
    used by a browser to identify itself -- some HTTP servers only
    allow requests coming from common browsers as opposed to scripts.
    For example, Mozilla Firefox may identify itself as ``"Mozilla/5.0
@@ -1111,6 +1111,9 @@ HTTPErrorProcessor Objects
 Examples
 --------
 
+In addition to the examples below, more examples are given in
+:ref:`urllib-howto`.
+
 This example gets the python.org main page and displays the first 300 bytes of
 it. ::
 
@@ -1220,6 +1223,8 @@ Use the *headers* argument to the :class:`Request` constructor, or::
    import urllib.request
    req = urllib.request.Request('http://www.example.com/')
    req.add_header('Referer', 'http://www.python.org/')
+   # Customize the default User-Agent header value:
+   req.add_header('User-Agent', 'urllib-example/0.1 (Contact: . . .)')
    r = urllib.request.urlopen(req)
 
 :class:`OpenerDirector` automatically adds a :mailheader:`User-Agent` header to