A file "CUserMedia.php" has been added in rev 18100 (2011/02/25) according a ZBXNEXT-629 development and there used only one function - get.
The get used in another API method user.get with selectMedias parameter.

Note that methods usermedia.* missing for 1.8 at all.

There even was an attempt to document it:
http://www.zabbix.com/documentation/2.0/manual/appendix/api/usermedia
http://www.zabbix.com/documentation/1.8/api/usermedia

Interesting, both 1.8 and 2.0 branches pages have been added 2010/08/09 14:15 - created aly (current)
Why they added when they was missing in the code? :/

So, please consider do remove usermedia.* methods (except .get) as redundant, but take a look, maybe there source code is better than currently in the user.addMedia() .deleteMedia() .updateMedia()

Please update documentation as well.