The documents are for vSphere, not for Horizon Linux Desktop.
Horizon Linux Desktop 6.2.0 has no support on copy/paste.
Horizon Linux Desktop 6.2.1 will be released soon, which has official copy/paste support on RHEL/CentOS 6.6 x64. For Ubuntu 1204/1404 x64, though there is no official copy/paste support, it should work.