V4L/DVB: v4l spec: document what odd and even fields are

Document that odd == top and even == bottom. I can never remember this,
so it is probably worth documenting.
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

Documentation/DocBook/v4l/io.xml

@@ -918,8 +918,8 @@ order</emphasis>.</para>
 
     <para>When the driver provides or accepts images field by field
 rather than interleaved, it is also important applications understand
-how the fields combine to frames. We distinguish between top and
-bottom fields, the <emphasis>spatial order</emphasis>: The first line
+how the fields combine to frames. We distinguish between top (aka odd) and
+bottom (aka even) fields, the <emphasis>spatial order</emphasis>: The first line
 of the top field is the first line of an interlaced frame, the first
 line of the bottom field is the second line of that frame.</para>
 
@@ -972,12 +972,12 @@ between <constant>V4L2_FIELD_TOP</constant> and
 	  <row>
 	    <entry><constant>V4L2_FIELD_TOP</constant></entry>
 	    <entry>2</entry>
-	    <entry>Images consist of the top field only.</entry>
+	    <entry>Images consist of the top (aka odd) field only.</entry>
 	  </row>
 	  <row>
 	    <entry><constant>V4L2_FIELD_BOTTOM</constant></entry>
 	    <entry>3</entry>
-	    <entry>Images consist of the bottom field only.
+	    <entry>Images consist of the bottom (aka even) field only.
 Applications may wish to prevent a device from capturing interlaced
 images because they will have "comb" or "feathering" artefacts around
 moving objects.</entry>