media: Documentation: v4l: Fix V4L2_CID_PIXEL_RATE documentation

The V4L2_CID_PIXEL_RATE is nowadays used to tell pixel sampling rate in
the sub-device's pixel array, not the pixel rate over a link (for which it
also becomes unfit with the addition of multiplexed streams later on). Fix
this.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Reviewed-by: Andrey Konovalov <andrey.konovalov@linaro.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>