Add api limitations in the documentation

docs/general.rst

@@ -15,3 +15,21 @@ JSON like that
         "status": 409,
         "message": "Conflict"
     }
+
+Limitations
+============
+
+Concurrency
+------------
+
+A VM can't process multiple request in the same time. But you can make
+multiple request on multiple VM. It's transparent for the client
+when the first request on a VM start a lock is acquire for this VM id
+and released for the next request at the end. You can safely send all
+the requests in the same time and let the server manage an efficent concurrency.
+
+We think it can be a little slower for some operations, but it's remove a big
+complexity for the client due to the fact only some command on some VM can be
+concurrent.
+
+