Update OpenAPI 3.0 known issues

During some testing today I noticed that the AWS Gateway does not except "anyOf" and "allOf" alongside "oneOf" for schema validation. See OpenAPI 3.0 documentation on data model validation for more details: https://swagger.io/docs/specification/data-models/oneof-anyof-allof-not/

Author: samclam23

doc_source/api-gateway-known-issues.md

@@ -35,7 +35,7 @@
   + The `example` tag is not supported\.
   + `exclusiveMinimum` is not supported by API Gateway\.
   + The `maxItems` and `minItems` tags are not included in simple request validation\. To work around this, update the model after import before doing validation\.
-  + `oneOf` is not supported by API Gateway\.
+  + `oneOf`, `anyOf`, and `allOf` are not supported by API Gateway\.
   + The `readOnly` field is not supported\.
   + Response definitions of the `"500": {"$ref": "#/responses/UnexpectedError"}` form is not supported in the OpenAPI document root\. To work around this, replace the reference by the inline schema\.
   + Numbers of the `Int32` or `Int64` type are not supported\. An example is shown as follows:
@@ -77,4 +77,4 @@
 +  The test invocation of a method uses the default content type of `application/json` and ignores specifications of any other content types\. 
 + When sending requests to an API by passing the `X-HTTP-Method-Override` header, API Gateway overrides the method\. So in order to pass the header to the backend, the header needs to be added to the integration request\.
 +  When a request contains multiple media types in its `Accept` header, API Gateway only honors the first `Accept` media type\. In the situation where you cannot control the order of the `Accept` media types and the media type of your binary content is not the first in the list, you can add the first `Accept` media type in the `binaryMediaTypes` list of your API, API Gateway will return your content as binary\. For example, to send a JPEG file using an `<img>` element in a browser, the browser might send `Accept:image/webp,image/*,*/*;q=0.8` in a request\. By adding `image/webp` to the `binaryMediaTypes` list, the endpoint will receive the JPEG file as binary\. 
-+ Customizing the default gateway response for `413 REQUEST_TOO_LARGE` isn't currently supported\.
++ Customizing the default gateway response for `413 REQUEST_TOO_LARGE` isn't currently supported\.