media: docs: uAPI: Clarify error documentation for invalid 'which' value

Invalid values for the 'which' field of structures passed to multiple
subdev ioctls result in an EINVAL error being returned. The
documentation of the corresponding ioctls indicates this with sentences
such as the following:

    the ``which`` field references a non-existing format

This is confusing. Clarify the documentation.

Suggested-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>

Documentation/userspace-api/media/v4l/vidioc-subdev-g-crop.rst

@@ -118,10 +118,9 @@ EBUSY
     ``VIDIOC_SUBDEV_S_CROP``
 
 EINVAL
-    The struct :c:type:`v4l2_subdev_crop` ``pad``
-    references a non-existing pad, the ``which`` field references a
-    non-existing format, or cropping is not supported on the given
-    subdev pad.
+    The struct :c:type:`v4l2_subdev_crop` ``pad`` references a non-existing pad,
+    the ``which`` field has an unsupported value, or cropping is not supported
+    on the given subdev pad.
 
 EPERM
     The ``VIDIOC_SUBDEV_S_CROP`` ioctl has been called on a read-only subdevice

Documentation/userspace-api/media/v4l/vidioc-subdev-g-fmt.rst

@@ -140,9 +140,8 @@ EBUSY
     fix the problem first. Only returned by ``VIDIOC_SUBDEV_S_FMT``
 
 EINVAL
-    The struct :c:type:`v4l2_subdev_format`
-    ``pad`` references a non-existing pad, or the ``which`` field
-    references a non-existing format.
+    The struct :c:type:`v4l2_subdev_format` ``pad`` references a non-existing
+    pad, or the ``which`` field has an unsupported value.
 
 EPERM
     The ``VIDIOC_SUBDEV_S_FMT`` ioctl has been called on a read-only subdevice

Documentation/userspace-api/media/v4l/vidioc-subdev-g-frame-interval.rst

@@ -117,10 +117,9 @@ EBUSY
     ``VIDIOC_SUBDEV_S_FRAME_INTERVAL``
 
 EINVAL
-    The struct
-    :c:type:`v4l2_subdev_frame_interval`
-    ``pad`` references a non-existing pad, the ``which`` field references a
-    non-existing frame interval, or the pad doesn't support frame intervals.
+    The struct :c:type:`v4l2_subdev_frame_interval` ``pad`` references a
+    non-existing pad, the ``which`` field has an unsupported value, or the pad
+    doesn't support frame intervals.
 
 EPERM
     The ``VIDIOC_SUBDEV_S_FRAME_INTERVAL`` ioctl has been called on a read-only

Documentation/userspace-api/media/v4l/vidioc-subdev-g-selection.rst

@@ -116,10 +116,9 @@ EBUSY
     ``VIDIOC_SUBDEV_S_SELECTION``
 
 EINVAL
-    The struct :c:type:`v4l2_subdev_selection`
-    ``pad`` references a non-existing pad, the ``which`` field
-    references a non-existing format, or the selection target is not
-    supported on the given subdev pad.
+    The struct :c:type:`v4l2_subdev_selection` ``pad`` references a
+    non-existing pad, the ``which`` field has an unsupported value, or the
+    selection target is not supported on the given subdev pad.
 
 EPERM
     The ``VIDIOC_SUBDEV_S_SELECTION`` ioctl has been called on a read-only