Replace AV1 still_picture=1 shoulds with a note (#228)

index.bs

@@ -174,9 +174,8 @@ When an item is of type <dfn export for="AV1 Image Item Type">av01</dfn>, it is
     - The content of an [=AV1 Image Item=] is called the <dfn export>AV1 Image Item Data</dfn> and shall obey the following constraints:
         - <assert>The [=AV1 Image Item Data=] shall be identical to the content of an [=AV1 Sample=] marked as 'sync', as defined in [[!AV1-ISOBMFF]].</assert>
         - <assert>The [=AV1 Image Item Data=] shall have exactly one [=Sequence Header OBU=].</assert>
-        - If the [=AV1 Image Item Data=] consists of a single frame (i.e. when using a single layer),
-            - <assert>It should have its <code>[=still_picture=]</code> flag set to 1.</assert>
-            - <assert>It should have its <code>[=reduced_still_picture_header=]</code> flag set to 1.</assert>
+
+NOTE: File writers may want to set the <code>[=still_picture=]</code> and <code>[=reduced_still_picture_header=]</code> flags to 1 when possible in the [=Sequence Header OBU=] part of the [=AV1 Image Item Data=] so that AV1 header overhead is minimized.
 
 <h3 id="image-item-properties">Image Item Properties</h3>
 
@@ -857,3 +856,4 @@ Other versions of the boxes shall not be used. "-" means that the box does not h
     - EDITORIAL: <a href="https://github.com/AOMediaCodec/av1-avif/pull/233">Update list of other item properties</a>
     - <a href="https://github.com/AOMediaCodec/av1-avif/pull/243">Further clarify relationship between av1C, metadata OBUs and item properties</a>
     - <a href="https://github.com/AOMediaCodec/av1-avif/pull/239">Add information on tmap, grpl and altr</a>
+    - <a href="https://github.com/AOMediaCodec/av1-avif/pull/228">Replace recommendations regarding still picture flags in image items by a note</a>