doc: migration-guide-4.2: video: add note on pitch

Document the change of semantics of video_format.pitch field,
modified in commit 33dcbe3.

Signed-off-by: Josuah Demangeon <me@josuah.net>

Author: josuah

doc/releases/migration-guide-4.2.rst

@@ -541,6 +541,12 @@ Video
   ``video_stream_start``
   ``video_stream_stop``
 
+* video_format.pitch is no longer to be set by the application, but instead written back by
+  the driver. Then the application can know how much padding this particular driver uses
+  per line, and in turn know the required buffer size. Existing applications will not be
+  broken by this change but can be simplified as performed in the sample in the commit
+  ``33dcbe37cfd3593e8c6e9cfd218dd31fdd533598``.
+
 Other subsystems
 ****************