bug: Fix OAuth Exchange docs

This was reported by a Customer.

When performing a private OAuth Exchange, the correct value for
`grant_type` is "authorization_code".  This parameter may also be
omitted, as this value is the default; it only need be provided (as
"refresh_token") when doing an OAuth Token Refresh.

Author: Dorthu

openapi.yaml

@@ -103,7 +103,7 @@ info:
 
     | PARAMETER | DESCRIPTION |
     |-----------|-------------|
-    | grant_type | The grant type you're using for renewal.  Currently only the string "refresh_token" is accepted. |
+    | grant_type | The grant type you're using for renewal.  Optional, only "authorization_code" is accepted for OAuth Exchanges. |
     | client_id | Your app's client ID. |
     | client_secret | Your app's client secret. |
     | code | The code you just received from the redirect. |