Adopt DM's suggested change to OpenAPI doc definition

Author: MikeRalphson

versions/3.1.0.md

@@ -68,7 +68,7 @@ An OpenAPI definition can then be used by documentation generation tools to disp
 ## Definitions
 
 ##### <a name="oasDocument"></a>OpenAPI Document
-A document (or set of documents) that defines or describes an API. An OpenAPI definition uses and conforms to the OpenAPI Specification.
+A document (or set of documents) that defines or describes an API or elements of an API. The document MUST contain at least one [paths](#pathsObject) field, a [components](#oasComponents) field or a [webhooks](#oasWebhooks) field. An OpenAPI definition uses and conforms to the OpenAPI Specification.
 
 ##### <a name="pathTemplating"></a>Path Templating
 Path templating refers to the usage of curly braces ({}) to mark a section of a URL path as replaceable using path parameters.