Added a bit more high-level documentation to android power monitors. Explained...

Added a bit more high-level documentation to android power monitors. Explained what ds2784 is and what android releases the data is available on.

BUG=

Review URL: https://codereview.chromium.org/652733002

Cr-Commit-Position: refs/heads/master@{#299311}

tools/telemetry/telemetry/core/platform/power_monitor/android_ds2784_power_monitor.py

@@ -19,6 +19,11 @@ VOLTAGE = os.path.join(FUEL_GAUGE_PATH, 'voltage_now')
 
 
 class DS2784PowerMonitor(sysfs_power_monitor.SysfsPowerMonitor):
+  """PowerMonitor that relies on the ds2784 fuel gauge chip.
+
+  The ds2784 chip is in some Android devices, for example the nexus 10.
+  This gives reliable energy numbers on Android L and higher releases.
+  """
   def __init__(self, device, platform_backend):
     super(DS2784PowerMonitor, self).__init__(platform_backend)
     self._device = device

tools/telemetry/telemetry/core/platform/power_monitor/android_dumpsys_power_monitor.py

@@ -13,6 +13,7 @@ class DumpsysPowerMonitor(sysfs_power_monitor.SysfsPowerMonitor):
   """PowerMonitor that relies on the dumpsys batterystats to monitor the power
   consumption of a single android application. This measure uses a heuristic
   and is the same information end-users see with the battery application.
+  Available on Android L and higher releases.
   """
   def __init__(self, device, platform_backend):
     """Constructor.