Documentation: thermal: document of_cpufreq_cooling_register()

Commit 39d99cff

 ("thermal: cpu_cooling: introduce
of_cpufreq_cooling_register") taught the cpu cooling device to register
devices that were linked to the device tree but didn't update the
cpu-cooling-api documentation.  Fix it.

Cc: Amit Daniel Kachhap <amit.kachhap@linaro.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Zhang Rui <rui.zhang@intel.com>
Cc: Eduardo Valentin <edubezval@gmail.com>
Signed-off-by: Javi Merino <javi.merino@arm.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>