PM / devfreq: Reword the kernel-doc comment for devfreq_monitor_start() API

Current kernel-doc comment doesn't specify the default timer used for the
load monitoring. Also, it uses the term "default delayed work" which could
be misunderstood as "default delayer timer". So reword the comment to
clearly specify the default timer and also reword the last sentence to make
it more understandable.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>