media: document MediaFormat.KEY_FRAME_RATE better
media: document MediaFormat.KEY_FRAME_RATE better

Bug: 27126548
Change-Id: I7e6a70bd192623d335816c1aa135448b7846aa92

xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base