[media] DocBook media: fix broken FIELD_ALTERNATE description

The sizeimage is that of a single field, not that of a full frame.
That makes no sense, and in fact all drivers supporting ALTERNATE will
set sizeimage to that of a field.
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>

Documentation/DocBook/media/v4l/io.xml

@@ -1470,10 +1470,9 @@ or application, depending on data direction, must set &v4l2-buffer;
 <constant>V4L2_FIELD_BOTTOM</constant>. Any two successive fields pair
 to build a frame. If fields are successive, without any dropped fields
 between them (fields can drop individually), can be determined from
-the &v4l2-buffer; <structfield>sequence</structfield> field. Image
-sizes refer to the frame, not fields. This format cannot be selected
-when using the read/write I/O method.<!-- Where it's indistinguishable
-from V4L2_FIELD_SEQ_*. --></entry>
+the &v4l2-buffer; <structfield>sequence</structfield> field. This format
+cannot be selected when using the read/write I/O method since there
+is no way to communicate if a field was a top or bottom field.</entry>
 	  </row>
 	  <row>
 	    <entry><constant>V4L2_FIELD_INTERLACED_TB</constant></entry>