Improve `Metric` doc

[llucax]

-        - This energy metric does not specify the timestamp since when the energy
-          was being accumulated, and therefore can be inconsistent.
+        - This energy metric does not specify the start time of the accumulation
+          period,and therefore can be inconsistent.

Originally posted by @shsms in frequenz-floss/frequenz-client-microgrid-python#158 (comment)