Settle on "OpenAPI Description" (3.0.4) (#3384)

This follows up from a discussion on the OAI slack that decided:

* redefining "document" to sometimes mean multiple documents is confusing
* "description" has more support than "definition"

versions/3.0.4.md

@@ -16,14 +16,14 @@ An OpenAPI definition can then be used by documentation generation tools to disp
 <!-- TOC depthFrom:1 depthTo:3 withLinks:1 updateOnSave:1 orderedList:0 -->
 
 - [Definitions](#definitions)
-	- [OpenAPI Document](#oasDocument)
+	- [OpenAPI Description](#oasDocument)
 	- [Path Templating](#pathTemplating)
 	- [Media Types](#mediaTypes)
 	- [HTTP Status Codes](#httpCodes)
 - [Specification](#specification)
 	- [Versions](#versions)
 	- [Format](#format)
-	- [Document Structure](#documentStructure)
+	- [OpenAPI Description Structure](#documentStructure)
 	- [Data Types](#dataTypes)
 	- [Rich Text Formatting](#richText)
 	- [Relative References In URLs](#relativeReferences)
@@ -67,8 +67,8 @@ 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 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.
 
 ##### <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.
@@ -135,11 +135,11 @@ In order to preserve the ability to round-trip between YAML and JSON formats, YA
 
 **Note:** While APIs may be defined by OpenAPI documents in either YAML or JSON format, the API request and response bodies and other content are not required to be JSON or YAML.
 
-### <a name="documentStructure"></a>Document Structure
+### <a name="documentStructure"></a>OpenAPI Description Structure
 
-An OpenAPI document MAY be made up of a single document or be divided into multiple, connected parts at the discretion of the user. In the latter case, `$ref` fields MUST be used in the specification to reference those parts as follows from the [JSON Schema](https://json-schema.org) definitions.
+An OpenAPI Description MAY be made up of a single document or be divided into multiple, connected parts at the discretion of the author. In the latter case, `$ref` fields MUST be used in the specification to reference those parts as follows from the [JSON Schema](https://json-schema.org) definitions.  In a multi-document description, the document containing the [OpenAPI Object](#oasObject) is known as the **entry OpenAPI document.**
 
-It is RECOMMENDED that the root OpenAPI document be named: `openapi.json` or `openapi.yaml`.
+It is RECOMMENDED that the entry OpenAPI document be named: `openapi.json` or `openapi.yaml`.
 
 ### <a name="dataTypes"></a>Data Types