[ci skip] Add notes about session locks for Redis, Memcached
diff --git a/user_guide_src/source/libraries/sessions.rst b/user_guide_src/source/libraries/sessions.rst
index c8a1f19..57c2585 100644
--- a/user_guide_src/source/libraries/sessions.rst
+++ b/user_guide_src/source/libraries/sessions.rst
@@ -630,6 +630,11 @@
Redis Driver
------------
+.. note:: Since Redis doesn't have a locking mechanism exposed, locks for
+ this driver are emulated by a separate value that is kept for up
+ to 5 seconds. You may experience issues if your page loads take
+ longer than that!
+
Redis is a storage engine typically used for caching and popular because
of its high performance, which is also probably your reason to use the
'redis' session driver.
@@ -663,6 +668,11 @@
Memcached Driver
----------------
+.. note:: Since Memcache doesn't have a locking mechanism exposed, locks
+ for this driver are emulated by a separate value that is kept for
+ up to 5 seconds. You may experience issues if your page loads take
+ longer than that!
+
The 'memcached' driver is very similar to the 'redis' one in all of its
properties, except perhaps for availability, because PHP's `Memcached
<http://php.net/memcached>`_ extension is distributed via PECL and some