samples: st: power_mgmt blinky: Minor rework and comments fixes

Remove mentions of st_counter_value which has been renamed to st_timeout.

Rework slightly the sample behavior to better demonstrate the interest
of st,timeout which allows to define an timeout matching exactly the
application needs.

Reword the various comments to make it hopefully clearer to the reader.

Signed-off-by: Erwan Gouriou <erwan.gouriou@st.com>