[media] V4L2 Spec: improve the G/S_INPUT/OUTPUT documentation

As Rupert pointed out, the phrase "It is good practice" should be replaced
with "You must".

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Cc: Rupert Eibauer <Rupert.Eibauer@ces.ch>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

Documentation/DocBook/media/v4l/vidioc-g-input.xml

@@ -61,8 +61,8 @@ desired input in an integer and call the
 <constant>VIDIOC_S_INPUT</constant> ioctl with a pointer to this
 integer. Side effects are possible. For example inputs may support
 different video standards, so the driver may implicitly switch the
-current standard. It is good practice to select an input before
-querying or negotiating any other parameters.</para>
+current standard. Because of these possible side effects applications
+must select an input before querying or negotiating any other parameters.</para>
 
     <para>Information about video inputs is available using the
 &VIDIOC-ENUMINPUT; ioctl.</para>

Documentation/DocBook/media/v4l/vidioc-g-output.xml

@@ -61,8 +61,9 @@ desired output in an integer and call the
 <constant>VIDIOC_S_OUTPUT</constant> ioctl with a pointer to this integer.
 Side effects are possible. For example outputs may support different
 video standards, so the driver may implicitly switch the current
-standard. It is good practice to select an output before querying or
-negotiating any other parameters.</para>
+standard.
+standard. Because of these possible side effects applications
+must select an output before querying or negotiating any other parameters.</para>
 
     <para>Information about video outputs is available using the
 &VIDIOC-ENUMOUTPUT; ioctl.</para>