CDRIVER-2229 link to docs for causal consistency

Author: ajdavis

doc/includes/session-lifecycle.txt

@@ -1,6 +1,6 @@
 Start a session with :symbol:`mongoc_client_start_session`, use the session for a sequence of operations, then free it with :symbol:`mongoc_client_session_destroy()`. A session must be freed before the :symbol:`mongoc_client_t` it came from.
 
-By default, sessions are causally consistent. To disable causal consistency, before starting a session create a :symbol:`mongoc_session_opt_t` with :symbol:`mongoc_session_opts_new()` and call :symbol:`mongoc_session_opts_set_causal_consistency()`, then free the struct with :symbol:`mongoc_session_opts_destroy`.
+By default, sessions are `causally consistent <http://dochub.mongodb.org/core/causal-consistency>`_. To disable causal consistency, before starting a session create a :symbol:`mongoc_session_opt_t` with :symbol:`mongoc_session_opts_new()` and call :symbol:`mongoc_session_opts_set_causal_consistency()`, then free the struct with :symbol:`mongoc_session_opts_destroy`.
 
 Unacknowledged writes are not causally consistent. If you execute a write operation with a :symbol:`mongoc_write_concern_t` on which you have called :symbol:`mongoc_write_concern_set_w` with a value of 0, the write does not participate in causal consistency.
 

doc/mongoc_client_session_t.rst

@@ -4,7 +4,7 @@
 mongoc_client_session_t
 =======================
 
-Use a session for a sequence of operations, optionally with causal consistency.
+Use a session for a sequence of operations, optionally with causal consistency. See `the MongoDB Manual Entry for Causal Consistency <http://dochub.mongodb.org/core/causal-consistency>`_.
 
 Synopsis
 --------

doc/mongoc_session_opts_set_causal_consistency.rst

@@ -12,7 +12,7 @@ Synopsis
   mongoc_session_opts_set_causal_consistency (mongoc_session_opt_t *opts,
                                               bool causal_consistency);
 
-Configure causal consistency in a session. If true (the default), each operation in the session will be causally ordered after the previous read or write operation. Set to false to disable causal consistency.
+Configure causal consistency in a session. If true (the default), each operation in the session will be causally ordered after the previous read or write operation. Set to false to disable causal consistency. See `the MongoDB Manual Entry for Causal Consistency <http://dochub.mongodb.org/core/causal-consistency>`_.
 
 Unacknowledged writes are not causally consistent. If you execute a write operation with a :symbol:`mongoc_write_concern_t` on which you have called :symbol:`mongoc_write_concern_set_w` with a value of 0, the write does not participate in causal consistency.