feat(AIP-193) Clarify Code usage in proto vs json (#1397)

Adds a callout of type "Important" that explains that `code` in
google.rpc.Status is the numeric equivalent of the proto enum value
found in `google.rpc.Code` while in JSON, it is the equivalent HTTP
status.

Closes #1396

Author: shwoodard

aip/general/0193.md

@@ -138,6 +138,18 @@ descriptions, and examples:
     `ErrorInfo`,`LocalizedMessage`, and so on.
   - *Example*: Described in the [ErrorDetails](#error-details) section.
 
+
+**Important:** In the context of the`google.rpc.Status` protobuf
+message, the value of the field `code` is the numeric equivalent to the
+enum value chosen from [`google.rpc.Code`][Code]. For example, if
+[`INVALID_ARGUMENT`][InvalidArgument] is chosen, the value of `code`
+will be `3` (in the context of the Protobuf message,
+`google.rpc.Status`). However, when the error is expressed as JSON, as
+in the [sample above](#json-representation), the value of the field by
+the same name, `"code"`, is the HTTP status code equivalent of the
+selected `google.rpc.Code`. For example, the value of the field `"code"`
+in the JSON reply would be `400`.
+
 #### [ErrorDetails][details]
 
 Google defines a set of [standard detail payloads][details] for error
@@ -538,5 +550,6 @@ data instead.
 [Error model]: https://cloud.google.com/apis/design/errors#error_model
 [Help]: https://github.com/googleapis/googleapis/blob/master/google/rpc/error_details.proto#L256
 [resource-error]: https://cloud.google.com/compute/docs/resource-error
+[InvalidArgument]: https://github.com/googleapis/googleapis/blob/d4acb64370d333024a167551e7da854506109ba2/google/rpc/code.proto#L52-L58
 
 <!-- prettier-ignore-end -->