Change non-layer-specific OBUs "must" to "should"

For backward compatibility, downgrade the requirement

  OBUs that are not layer-specific must have the obu_extension_flag
  set to 0.

to a recommendation

  OBUs that are not layer-specific should have the obu_extension_flag
  set to 0.

07.bitstream.semantics.md

@@ -81,7 +81,7 @@ possible by preventing emulation of MPEG2 transport stream ids.
 Reserved units are for future use and shall be ignored by AV1 decoder.
 
 The column “Layer-specific” indicates if the corresponding OBU type is considered to be associated with
-a specific layer  ("Y"), or not ("N"). OBUs that are not layer-specific must have the obu_extension_flag set to 0. 
+a specific layer  ("Y"), or not ("N"). OBUs that are not layer-specific should have the obu_extension_flag set to 0.
 
 Metadata OBU types may or may not be layer-specific, depending on the metadata type. The table in Section 6.7.1 specifies which types of metadata OBUs are layer-specific and which are not.