Update `replication.md` with info on TCP module structure (#12621)

changelog.d/12621.doc

@@ -0,0 +1 @@
+Add information about the TCP replication module to docs.

docs/replication.md

@@ -35,3 +35,8 @@ See [the TCP replication documentation](tcp_replication.md).
 There are read-only version of the synapse storage layer in
 `synapse/replication/slave/storage` that use the response of the
 replication API to invalidate their caches.
+
+### The TCP Replication Module
+Information about how the tcp replication module is structured, including how
+the classes interact, can be found in
+`synapse/replication/tcp/__init__.py`

synapse/replication/tcp/__init__.py

@@ -15,7 +15,7 @@
 """This module implements the TCP replication protocol used by synapse to
 communicate between the master process and its workers (when they're enabled).
 
-Further details can be found in docs/tcp_replication.rst
+Further details can be found in docs/tcp_replication.md
 
 
 Structure of the module: