[C++] Document connection pool vs process fork order (#100)

* add fork safety

Author: rachel-mack

source/connect/connection-pools.txt

@@ -29,14 +29,21 @@ rather than "stopping the world", it's highly recommended to use a
 application has access to multiple threads, even if your application only
 uses one thread.
 
-Connection Pools and Thread Safety
-----------------------------------
+Using Threads with Connection Pools
+-----------------------------------
 
 A ``mongocxx::pool`` can be shared across multiple threads and used to create
 clients. However, each ``mongocxx::client`` can only be used in a single
 thread. See the :ref:`thread safety documentation <cpp-thread-safety>` for
 details on how to use a ``mongocxx::client`` in a thread-safe manner.
 
+Using Forks with Connection Pools
+----------------------------------
+
+A ``mongocxx::pool`` **cannot** be shared between a parent and a fork.
+You must create your connection pool after forking. See the :ref:`fork safety
+documentation <cpp-fork-safety>` documentation. 
+
 Configuring a Connection Pool
 -----------------------------
 

source/thread-safety.txt

@@ -117,6 +117,8 @@ In most programs, clients will be long lived for convenience and performance. In
 there's quite a bit of overhead because we're doing so little work with each
 client, but typically this is the best solution.
 
+.. _cpp-fork-safety:
+
 Fork Safety
 -----------