Document asyncio's TLS read buffer.

Fix #1644.

Author: aaugustin

docs/topics/memory.rst

@@ -88,7 +88,9 @@ the frequency of system calls or the need to pause reading or writing.
 You should keep these buffers small. Increasing them can help with spiky
 workloads but it can also backfire because it delays backpressure.
 
-* In the :mod:`asyncio` implementation, there is no library-level read buffer.
+* In the :mod:`asyncio` implementation, there is no library-level read buffer
+  for plain TCP connections. There is a 256 KiB read buffer for TLS connections,
+  plus 32KiB on Windows with the default event loop.
 
   There is a write buffer. The ``write_limit`` argument of
   :func:`~asyncio.client.connect` and :func:`~asyncio.server.serve` controls its
@@ -107,6 +109,9 @@ workloads but it can also backfire because it delays backpressure.
   it drains under the low-water mark. This creates backpressure on the TCP
   connection.
 
+  In addition, there is a 256 KiB read buffer for TLS connections, plus 32KiB on
+  Windows.
+
   There is a write buffer. It as controlled by ``write_limit``. It behaves like
   the :mod:`asyncio` implementation described above.
 

docs/topics/performance.rst

@@ -5,6 +5,13 @@ Performance
 
 Here are tips to optimize performance.
 
+TLS
+---
+
+You should terminate TLS in a reverse proxy rather than in a websockets server.
+
+This will reduce memory usage of the websockets server and improve performance.
+
 uvloop
 ------