Clarify empty Security Requirement Object usage and validity (OAI#1886)

* Clarify empty Security Requirement Object usage and validity

* Reorder sentences to make clearer.

* Remove wrong text.

* Removed unneeded text.

Co-authored-by: Ron <ron@swagger.io>
Signed-off-by: Mike Ralphson <mike.ralphson@gmail.com>

versions/3.1.0.md

@@ -195,7 +195,7 @@ Field Name | Type | Description
 <a name="oasPaths"></a>paths | [Paths Object](#pathsObject) | **REQUIRED**. The available paths and operations for the API.
 <a name="oasWebhooks"></a>webhooks | Map[`string`, [Path Item Object](#pathItemObject)] | The incoming webhooks that MAY be received as part of this API and that the API consumer MAY choose to implement. Closely related to the `callbacks` feature, this section describes requests initiated other than by an API call, for example by an out of band registration. The key name is a unique string to refer to each webhook, while the Path Item Object describes a request that may be initiated by the API provider and the expected responses. An [example](../examples/v3.1/webhook-example.yaml) is available.
 <a name="oasComponents"></a>components | [Components Object](#componentsObject) | An element to hold various schemas for the specification.
-<a name="oasSecurity"></a>security | [[Security Requirement Object](#securityRequirementObject)] | A declaration of which security mechanisms can be used across the API. The list of values includes alternative security requirement objects that can be used. Only one of the security requirement objects need to be satisfied to authorize a request. Individual operations can override this definition.
+<a name="oasSecurity"></a>security | [[Security Requirement Object](#securityRequirementObject)] | A declaration of which security mechanisms can be used across the API. The list of values includes alternative security requirement objects that can be used. Only one of the security requirement objects need to be satisfied to authorize a request. Individual operations can override this definition. To make security optional, an empty security requirement (`{}`) can be included in the array.
 <a name="oasTags"></a>tags | [[Tag Object](#tagObject)] | A list of tags used by the specification with additional metadata. The order of the tags can be used to reflect on their order by the parsing tools. Not all tags that are used by the [Operation Object](#operationObject) must be declared. The tags that are not declared MAY be organized randomly or based on the tools' logic. Each tag name in the list MUST be unique.
 <a name="oasExternalDocs"></a>externalDocs | [External Documentation Object](#externalDocumentationObject) | Additional external documentation.
 
@@ -855,7 +855,7 @@ Field Name | Type | Description
 <a name="operationResponses"></a>responses | [Responses Object](#responsesObject) | **REQUIRED**. The list of possible responses as they are returned from executing this operation.
 <a name="operationCallbacks"></a>callbacks | Map[`string`, [Callback Object](#callbackObject) \| [Reference Object](#referenceObject)] | A map of possible out-of band callbacks related to the parent operation. The key is a unique identifier for the Callback Object. Each value in the map is a [Callback Object](#callbackObject) that describes a request that may be initiated by the API provider and the expected responses.
 <a name="operationDeprecated"></a>deprecated | `boolean` | Declares this operation to be deprecated. Consumers SHOULD refrain from usage of the declared operation. Default value is `false`.
-<a name="operationSecurity"></a>security | [[Security Requirement Object](#securityRequirementObject)] | A declaration of which security mechanisms can be used for this operation. The list of values includes alternative security requirement objects that can be used. Only one of the security requirement objects need to be satisfied to authorize a request. This definition overrides any declared top-level [`security`](#oasSecurity). To remove a top-level security declaration, an empty array can be used.
+<a name="operationSecurity"></a>security | [[Security Requirement Object](#securityRequirementObject)] | A declaration of which security mechanisms can be used for this operation. The list of values includes alternative security requirement objects that can be used. Only one of the security requirement objects need to be satisfied to authorize a request. To make security optional, an empty security requirement (`{}`) can be included in the array. This definition overrides any declared top-level [`security`](#oasSecurity). To remove a top-level security declaration, an empty array can be used.
 <a name="operationServers"></a>servers | [[Server Object](#serverObject)] | An alternative `server` array to service this operation. If an alternative `server` object is specified at the Path Item Object or Root level, it will be overridden by this value.
 
 This object MAY be extended with [Specification Extensions](#specificationExtensions).
@@ -3367,6 +3367,32 @@ petstore_auth:
 - read:pets
 ```
 
+###### Optional OAuth2 Security
+
+Optional OAuth2 security as would be defined in an <a href="#openapi-object">OpenAPI Object</a> or an <a href="#operation-object">Operation Object</a>:
+
+```json
+{
+  "security": [
+    {},
+    {
+      "petstore_auth": [
+        "write:pets",
+        "read:pets"
+      ]
+    }
+  ]
+}
+```
+
+```yaml
+security:
+  - {}
+  - petstore_auth:
+    - write:pets
+    - read:pets
+```
+
 ### <a name="specificationExtensions"></a>Specification Extensions
 
 While the OpenAPI Specification tries to accommodate most use cases, additional data can be added to extend the specification at certain points.