Changed documentation on eventlet (celery#4724)

* This is redundant because symbol_by_name will be called for self.registry_clss in __init__ method

* Corrected 'blocking call' to 'CPU-bound operation'

* additional changes in the comment after review

docs/userguide/concurrency/eventlet.rst

@@ -21,10 +21,16 @@ change how you run your code, not how you write it.
     * The event dispatch is implicit: meaning you can easily use Eventlet
       from the Python interpreter, or as a small part of a larger application.
 
-Celery supports Eventlet as an alternative execution pool implementation.
-It's in some cases superior to prefork, but you need to ensure
-your tasks don't perform blocking calls, as this will halt all
-other operations in the worker until the blocking call returns.
+
+Celery supports Eventlet as an alternative execution pool implementation and
+in some cases superior to prefork. However, you need to ensure one task doesn't
+block the event loop too long. Generally, CPU-bound operations don't go well
+with Evenetlet. Also note that some libraries, usually with C extensions,
+cannot be monkeypatched and therefore cannot benefit from using Eventlet.
+Please refer to their documentation if you are not sure. For example, pylibmc
+does not allow cooperation with Eventlet but psycopg2 does when both of them
+are libraries with C extensions.
+
 
 The prefork pool can take use of multiple processes, but how many is
 often limited to a few processes per CPU. With Eventlet you can efficiently