message: Update comment fields of AudioPassThru capabilities in MOBILE_API.xml
diff: <enum name="SamplingRate">
  <description>Describes different sampling options for PerformAudioPassThru.</description>
  - <element name="8KHZ" internal_name="SamplingRate_8KHZ"/>
  - <element name="16KHZ" internal_name="SamplingRate_16KHZ"/>
  - <element name="22KHZ" internal_name="SamplingRate_22KHZ"/>
  - <element name="44KHZ" internal_name="SamplingRate_44KHZ"/>
  + <element name="8KHZ" internal_name="SamplingRate_8KHZ">
  + <description>Sampling rate of 8000 Hz.</description>
  + </element>
  + <element name="16KHZ" internal_name="SamplingRate_16KHZ">
  + <description>Sampling rate of 16000 Hz.</description>
  + </element>
  + <element name="22KHZ" internal_name="SamplingRate_22KHZ">
  + <description>Sampling rate of 22050 Hz.</description>
  + </element>
  + <element name="44KHZ" internal_name="SamplingRate_44KHZ">
  + <description>Sampling rate of 44100 Hz.</description>
  + </element>
  </enum>
  <enum name="BitsPerSample">
  <description>Describes different quality options for PerformAudioPassThru.</description>
  - <element name="8_BIT" internal_name="BitsPerSample_8_BIT"/>
  - <element name="16_BIT" internal_name="BitsPerSample_16_BIT"/>
  + <element name="8_BIT" internal_name="BitsPerSample_8_BIT">
  + <description>Audio sample is 8 bits wide, unsigned.</description>
  + </element>
  + <element name="16_BIT" internal_name="BitsPerSample_16_BIT">
  + <description>Audio sample is 16 bits wide, signed, and in little endian.</description>
  + </element>
  </enum>
  <enum name="AudioType">
  <description>Describes different audio type options for PerformAudioPassThru.</description>
  - <element name="PCM" />
  + <element name="PCM">
  + <description>Linear PCM.</description>
  + </element>
  </enum>
  <struct name="AudioPassThruCapabilities">
  <description>
  Describes different audio type configurations for PerformAudioPassThru.
  e.g. {8kHz,8-bit,PCM}
  + The audio is recorded in monaural.
  </description>
  <param name="samplingRate" type="SamplingRate" mandatory="true"/>
  <param name="bitsPerSample" type="BitsPerSample" mandatory="true"/>
  