[Schema] Clarify that not required -> optional

Signed-off-by: Rob Dolin <[email protected]>

Author: RobDolinMS

versions/3.0.md

@@ -175,6 +175,8 @@ Relative references used in $ref are processed as per [JSON Reference](https://t
 
 ### Schema
 
+In the following description, if an object is not explicitly **Required** or described with a MUST or SHALL, it can be considered OPTIONAL.
+
 #### <a name="oasObject"></a>OpenAPI Object
 
 This is the root document object for the API specification.