Documentation: media: move Memory Consistency Flags

The documentation of the Memory Consistency Flags was part of the struct
v4l2_buffer documentation, but that struct doesn't use those flags. Instead
it is used by VIDIOC_CREATE_BUFS and VIDIOC_REQBUFS. Move the documentation
from buffer.rst to vidioc-reqbufs.rst which is where it belongs.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>