Add clarification on multithreading and multiprocessing for resources

Addresses boto/botocore#1246

docs/source/guide/resources.rst

@@ -199,22 +199,32 @@ keyword arguments. Examples of waiters include::
 
 
 Multithreading and multiprocessing
---------------------------------
-It is recommended to create a resource instance for each thread / process in a multithreaded or multiprocess application rather than sharing a single instance among the threads / processes. For example::
+----------------------------------
+SDK object instances are **not** thread safe in almost all cases and should not be shared across threads or processes. You should create a new SDK object for each thread or process::
 
     import boto3
     import boto3.session
     import threading
 
     class MyTask(threading.Thread):
         def run(self):
+            # Here we create a new session per thread
             session = boto3.session.Session()
+
+            # Next, we create a resource client using our thread's session object
             s3 = session.resource('s3')
-            # ... do some work with S3 ...
+
+            # Put your thread-safe code here
 
 In the example above, each thread would have its own Boto3 session and its own instance of the S3 resource. This is a good idea because resources contain shared data when loaded and calling actions, accessing properties, or manually loading or reloading the resource can modify this data.
 
 .. note::
-    Resources are **not** thread safe. These special classes contain additional meta data that cannot be shared between threads. When using a Resource, it is recommended to instantiate a new Resource for each thread, as is shown in the example above. 
-
-    Low-level clients **are** thread safe. When using a low-level client, it is recommended to instantiate your client then pass that client object to each of your threads. 
+    Resources are **not** thread safe. These special classes contain additional meta data that cannot be shared between threads. When using a Resource, it is recommended to instantiate a new Resource for each thread, as is shown in the example above.
+
+    Sessions are **not** thread safe. This is due to shared meta data, similar to Resource.
+
+    Low-level clients **are** thread safe in a **threaded environment** in all cases when using the client's attached operation methods (i.e. ``s3.get_object``). However, clients are **not** thread safe if you are explicitly using the client to interact with the underlying Session (i.e. using ``s3.meta``, ``s3.exceptions``, or ``s3.waiter_names``).
+
+    Low-level clients are **not** safe in a **multiprocess environment**. This is due to `forking issues <https://github.com/psf/requests/issues/4323>`__ with urllib3's connection pool.
+
+