reqRequestBody doc update

Yesod/Request.hs

@@ -100,6 +100,13 @@ data Request = Request
       -- thunk, which essentially means it will be computed once at most, but
       -- only if requested. This allows avoidance of the potentially costly
       -- parsing of POST bodies for pages which do not use them.
+      --
+      -- Additionally, since the request body is not read until needed, you can
+      -- directly access the 'W.requestBody' record in 'reqWaiRequest' and
+      -- perform other forms of parsing. For example, when designing a web
+      -- service, you may want to accept JSON-encoded data. Just be aware that
+      -- if you do such parsing, the standard POST form parsing functions will
+      -- no longer work.
     , reqRequestBody :: IO RequestBodyContents
     , reqWaiRequest :: W.Request
       -- | Languages which the client supports.