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>

Documentation/userspace-api/media/v4l/ext-ctrls-image-process.rst

@@ -40,7 +40,7 @@ Image Process Control IDs
 .. _v4l2-cid-pixel-rate:
 
 ``V4L2_CID_PIXEL_RATE (64-bit integer)``
-    Pixel rate in the source pads of the subdev. This control is
+    Pixel sampling rate in the device's pixel array. This control is
     read-only and its unit is pixels / second.
 
 ``V4L2_CID_TEST_PATTERN (menu)``