Backport improved OpenAPI specification intro paragraph from 3.1.1 (original PR OAI#3387)

Author: lornajane

versions/3.0.4.md

@@ -68,7 +68,7 @@ An OpenAPI definition can then be used by documentation generation tools to disp
 ## Definitions
 
 ##### <a name="oasDocument"></a>OpenAPI Description
-A document (or set of documents) that describes an API. An OpenAPI Description (OAD) uses and conforms to the OpenAPI Specification.
+An OpenAPI Description (OAD) formally describes the surface of an API and its semantics. It is composed of an [entry document](#documentStructure) and any/all of its referenced documents. An OAD uses and conforms to the OpenAPI Specification.
 
 ##### <a name="pathTemplating"></a>Path Templating
 Path templating refers to the usage of template expressions, delimited by curly braces ({}), to mark a section of a URL path as replaceable using path parameters.