Clarify how to model streaming binary data (3.1.1)

Also make the translation from 3.0 binary modeling a subsection,
as otherwise the line about streaming either feels randomly inserted
into the middle of the section, or feels disconnected when placed
after the conversion table.

versions/3.1.1.md

@@ -196,6 +196,9 @@ The `contentMediaType` keyword is redundant if the media type is already set:
 
 If the Schema Object will be processed by a non-OAS-aware JSON Schema implementation, it may be useful to include `contentMediaType` even if it is redundant.  However, if `contentMediaType` contradicts a relevant Media Type Object or Encoding Object, then `contentMediaType` SHALL be ignored.
 
+The `maxLength` keyword MAY be used to set an expected upper bound on the length of a streaming payload.  The keyword can be applied to either string data, including encoded binary data, or to unencoded binary data.  For unencoded binary, the length is the number of octets.
+
+##### Migrating binary descriptions from OAS 3.0
 The following table shows how to migrate from OAS 3.0 binary data descriptions, continuing to use `image/png` as the example binary media type:
 
 OAS < 3.1 | OAS 3.1 | Comments