Update oauth documentation

authentication/POST_oauth_accesstoken.md

@@ -3,14 +3,16 @@
     POST oauth/access_token
 
 ## Description
-Allows a Consumer application to obtain an OAuth Request Token to request user authorization. This method fulfills [Section 6.1][] of the [OAuth 1.0 authentication flow][].
+Allows a Consumer application to exchange an OAuth Request Token for an OAuth Access Token, which can be used to access protected resources on behalf of the user. This method fulfills [Section 6.3][] of the [OAuth 1.0 authentication flow][].
 
-The OAuth access token may also be used for xAuth operations.
+The OAuth Access Token may also be used for xAuth operations.
 
 ***
 
 ## Requires authentication
-OAuth Request Token received using the **[request_token][]** method. Requires **oauth_verifier** when using **[authorize](https://github.com/500px/api-documentation/blob/master/authentication/POST_oauth_authorize.modificator)** in authentication workflow.
+OAuth Request Token received using the [request_token][] method. Requires oauth_verifier when using [authorize](https://github.com/500px/api-documentation/blob/master/authentication/POST_oauth_authorize.modificator) in authentication workflow.
+
+Note that this request must be signed with a Request Token even when using the xAuth workflow.
 
 ***
 
@@ -39,6 +41,6 @@ Text, containing an query string-encoded list of OAuth parameters.
 
 - **401 Invalid OAuth Request** — Request for the token was malformed or uses an unknown OAuth version
 
-[Section 6.1]: http://oauth.net/core/1.0/#auth_step1
+[Section 6.3]: http://oauth.net/core/1.0/#auth_step3
 [OAuth 1.0 authentication flow]: http://oauth.net/core/1.0/#anchor9
-[request_token]: https://github.com/500px/api-documentation/blob/master/authentication/POST_oauth_requesttoken.md
+[request_token]: https://github.com/500px/api-documentation/blob/master/authentication/POST_oauth_requesttoken.md