update FAQ to include disabling graphics (#4159)

* update FAQ and exception message

* remove colab link

docs/FAQ.md

   Unity Environment to figure what error happened.
 - _Cause_: You have assigned `HTTP_PROXY` and `HTTPS_PROXY` values in your
   environment variables. _Solution_: Remove these values and try again.
+- _Cause_: You are running in a headless environment (e.g. remotely connected
+  to a server). _Solution_: Pass `--no-graphics` to `mlagents-learn`, or
+  `no_graphics=True` to `RemoteRegistryEntry.make()` or the `UnityEnvironment`
+  initializer. If you need graphics for visual observations, you will need to
+  set up `xvfb` (or equivalent).
 
 ## Communication port {} still in use
 

ml-agents-envs/mlagents_envs/rpc_communicator.py

             raise UnityTimeOutException(
                 "The Unity environment took too long to respond. Make sure that :\n"
                 "\t The environment does not need user interaction to launch\n"
-                "\t The Agents are linked to the appropriate Brains\n"
+                '\t The Agents\' Behavior Parameters > Behavior Type is set to "Default"\n'
                 "\t The environment and the Python interface have compatible versions."
             )