doc: python bindings howto

* Added a new user ID for Danger Mouse.
* Removed the empty entry for key preferences since that is handled
  through gpg.conf and/or editing the key directly.

lang/python/docs/GPGMEpythonHOWTOen.org

@@ -1251,16 +1251,43 @@
    :CUSTOM_ID: keygen-uids
    :END:
 
+   By comparison to creating primary keys and subkeys, adding a new
+   user ID to an existing key is much simpler.  The method used to do
+   this is =key_add_uid= and the only arguments it takes are for the
+ =key= and the new =uid=.
 
-** Key preferences
+   #+begin_src python
-   :PROPERTIES:
+     import gpg
-   :CUSTOM_ID: keygen-prefs
+
-   :END:
+     c = gpg.Context()
+     c.home_dir = "/tmp/dmgpg"
+
+     dmfpr = "177B7C25DB99745EE2EE13ED026D2F19E99E63AA"
+     key = c.get_key(dmfpr, secret = True)
+     uid = "Danger Mouse <danger.mouse@secret.example.net>"
+
+     c.key_add_uid(key, uid)
+   #+end_src
+
+   Unsurprisingly the result of this is:
+
+   #+begin_src shell
+     bash-4.4$ gpg --homedir /tmp/dmgpg -K
+     /tmp/dmgpg/pubring.kbx
+     ----------------------
+     sec   rsa3072 2018-03-15 [SC] [expires: 2019-03-15]
+	   177B7C25DB99745EE2EE13ED026D2F19E99E63AA
+     uid           [ultimate] Danger Mouse <danger.mouse@secret.example.net>
+     uid           [ultimate] Danger Mouse <dm@secret.example.net>
+     ssb   rsa3072 2018-03-15 [E] [expires: 2018-09-13]
+
+     bash-4.4$
+   #+end_src
 
 
 ** Key certification
    :PROPERTIES:
-   :CUSTOM_ID: keygen-certify
+   :CUSTOM_ID: key-sign
    :END: