[media] v4l: Correct conflicting V4L2 subdev selection API documentation

The API reference documents that the KEEP_CONFIG flag tells the
configuration should not be propagated by the driver whereas the interface
documentation (dev-subdev.xml) prohibited any changes to the rest of the
pipeline. Resolve the conflict by changing the API reference to disallow
changes.

Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Acked-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

Documentation/DocBook/media/v4l/selections-common.xml

@@ -148,7 +148,7 @@
 	  <row>
 	    <entry><constant>V4L2_SEL_FLAG_KEEP_CONFIG</constant></entry>
 	    <entry>(1 &lt;&lt; 2)</entry>
-	    <entry>The configuration should not be propagated to any
+	    <entry>The configuration must not be propagated to any
 	    further processing steps. If this flag is not given, the
 	    configuration is propagated inside the subdevice to all
 	    further processing steps.</entry>