Clarify constraints on Security Scheme Object Scheme Property (OAI#1880)

* Wording around scheme extensions

* Clarified that securitySchemeScheme is only a SHOULD be registered scheme

Signed-off-by: Mike Ralphson <mike.ralphson@gmail.com>

versions/3.1.0.md

@@ -3180,7 +3180,7 @@ Field Name | Type | Applies To | Description
 <a name="securitySchemeDescription"></a>description | `string` | Any | A short description for security scheme. [CommonMark syntax](https://spec.commonmark.org/) MAY be used for rich text representation.
 <a name="securitySchemeName"></a>name | `string` | `apiKey` | **REQUIRED**. The name of the header, query or cookie parameter to be used.
 <a name="securitySchemeIn"></a>in | `string` | `apiKey` | **REQUIRED**. The location of the API key. Valid values are `"query"`, `"header"` or `"cookie"`.
-<a name="securitySchemeScheme"></a>scheme | `string` | `http` | **REQUIRED**. The name of the HTTP Authorization scheme to be used in the [Authorization header as defined in RFC7235](https://tools.ietf.org/html/rfc7235#section-5.1).
+<a name="securitySchemeScheme"></a>scheme | `string` | `http` | **REQUIRED**. The name of the HTTP Authorization scheme to be used in the [Authorization header as defined in RFC7235](https://tools.ietf.org/html/rfc7235#section-5.1).  The values used SHOULD be registered in the [IANA Authentication Scheme registry](https://www.iana.org/assignments/http-authschemes/http-authschemes.xhtml).
 <a name="securitySchemeBearerFormat"></a>bearerFormat | `string` | `http` (`"bearer"`) | A hint to the client to identify how the bearer token is formatted.  Bearer tokens are usually generated by an authorization server, so this information is primarily for documentation purposes.
 <a name="securitySchemeFlows"></a>flows | [OAuth Flows Object](#oauthFlowsObject) | `oauth2` | **REQUIRED**. An object containing configuration information for the flow types supported.
 <a name="securitySchemeOpenIdConnectUrl"></a>openIdConnectUrl | `string` | `openIdConnect` | **REQUIRED**. OpenId Connect URL to discover OAuth2 configuration values. This MUST be in the form of a URL. The OpenID Connect standard requires the use of TLS.