add documentation to V4L2_CID_FWHT_I/P_FRAME_QP
controls in ext-ctrls-codec.rst

[mchehab+samsung@kernel.org: remove extra blank lines]
Signed-off-by: Dafna Hirschfeld <dafna3@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>