media: vidioc-prepare-buf.rst: drop reference to NO_CACHE flags

pass ownership of the buffer to the driver before actually enqueuing it,

using the :ref:`VIDIOC_QBUF <VIDIOC_QBUF>` ioctl, and to prepare it for future I/O. Such

preparations may include cache invalidation or cleaning. Performing them

in advance saves time during the actual I/O. In case such cache

operations are not required, the application can use one of

``V4L2_BUF_FLAG_NO_CACHE_INVALIDATE`` and

``V4L2_BUF_FLAG_NO_CACHE_CLEAN`` flags to skip the respective step.

in advance saves time during the actual I/O.

The struct :c:type:`v4l2_buffer` structure is specified in

:ref:`buffer`.