Chromium: Add SensorDevice::SetFrequency comments of Pausing and Resuming

This commit adds comments of pausing and resuming the reading with
chromeos::sensors::SensorDevice::SetFrequency.

BUG=chromium:1006141
TEST=none

Change-Id: Iffbafdae7f8091d121a0beed3cca752d59236194
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2470286Reviewed-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
Reviewed-by: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Commit-Queue: Cheng-Hao Yang <chenghaoyang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#817336}

chromeos/components/sensors/mojom/sensor.mojom

@@ -75,7 +75,12 @@ interface SensorDevice {
   // Returns base::nullopt if the attribute cannot be read.
   GetAttribute@1(string attr_name) => (string? value);
 
-  // Sets the frequency of the device before starting to read samples.
+  // Sets the frequency in Hz of the device before starting to read samples.
+  // If no frequency or an invalid frequency is set, an
+  // ObserverErrorType::FREQUENCY_INVALID error will occur after calling
+  // |StartReadingSamples|.
+  // After starting to read samples, the client can set the frequency to 0 to
+  // pause the reading, and set a valid frequency later to resume the reading.
   // |result_freq| would be the frequency set by iioservice for the client.
   SetFrequency@2(double frequency) => (double result_freq);