Clarify type and contents for payload

Remove confusing duplication of referencing JSON, XML and TEXT, the caps imply the "type" rather than the contents of the string. 

Given the contents can be text not really sure what the point of the saying it has to be well formed json, xml or text. If that is, because the type is checked and the content-type set according to what is determined. The it should say so

Author: simonsabin

docs/relational-databases/system-stored-procedures/sp-invoke-external-rest-endpoint-transact-sql.md

@@ -66,7 +66,7 @@ URL of the HTTPS REST endpoint to be called. *@url* is **nvarchar(4000)** with n
 
 #### [ @payload = ] N'*request_payload*'
 
-Unicode string in a JSON, XML, or TEXT format that contains the payload to send to the HTTPS REST endpoint. Payloads must be a valid JSON document, a well formed XML document, or text. *@payload* is **nvarchar(max)** with no default.
+The payload to send to the HTTPS REST endpoint. Must be a valid JSON document, a well formed XML document, or text. *@payload* is **nvarchar(max)** type with no default.
 
 #### [ @headers = ] N'*headers*'