message: Update mobile api to include description
diff: <enum name="ButtonName" since="1.0">
  <description>Defines the hard (physical) and soft (touchscreen) buttons available from the module</description>
  <element name="OK" />
  - <element name="PLAY_PAUSE" since="5.0"/>
  + <element name="PLAY_PAUSE" since="5.0">
  + <description>
  + The button name for the physical Play/Pause
  + toggle that can be used by media apps.
  + </description>
  + <warning>
  + Please use the physical OK button in order to
  + use a Play/Pause toggle for versions &lt; 4.5.0.
  + </warning>
  + </element>
  <element name="SEEKLEFT" />
  <element name="SEEKRIGHT" />
  <element name="TUNEUP" />
  