Update createInputSurface() doc

The Surface returned by MediaCodec#createInputSurface() is configured
for HW_VIDEO_ENCODER by the consumer.  Accessing the Surface through
Surface#lockCanvas() sets SW_READ_OFTEN | SW_WRITE_OFTEN, and some
devices choke on the combination.

Bug 11418094

Change-Id: Ic182802386146d19b1bdecdb894c5130c563a3ed