diff --git a/doc/source/plugins.rst b/doc/source/plugins.rst
index 8bd3797cd2..b8da7e1237 100644
--- a/doc/source/plugins.rst
+++ b/doc/source/plugins.rst
@@ -19,7 +19,16 @@ DevStack supports a standard mechanism for including plugins from
 external repositories. The plugin interface assumes the following:
 
 An external git repository that includes a ``devstack/`` top level
-directory. Inside this directory there can be 2 files.
+directory. Inside this directory there can be 3 files.
+
+- ``override_defaults`` - a file containing global variables that
+  will be sourced before the lib/* files. This allows the plugin
+  to override the defaults that are otherwise set in the lib/*
+  files.
+
+  For example, override_defaults may export CINDER_ENABLED_BACKENDS
+  to include the plugin-specific storage backend and thus be able
+  to override the default lvm only storage backend for Cinder.
 
 - ``settings`` - a file containing global variables that will be
   sourced very early in the process. This is helpful if other plugins
@@ -38,7 +47,7 @@ directory. Inside this directory there can be 2 files.
 
 - ``plugin.sh`` - the actual plugin. It is executed by devstack at
   well defined points during a ``stack.sh`` run. The plugin.sh
-  internal structure is discussed bellow.
+  internal structure is discussed below.
 
 
 Plugins are registered by adding the following to the localrc section
diff --git a/extras.d/README.md b/extras.d/README.md
index 7c2e4fe970..4cec14b4e7 100644
--- a/extras.d/README.md
+++ b/extras.d/README.md
@@ -14,10 +14,13 @@ The scripts are sourced at the beginning of each script that calls them. The
 entire `stack.sh` variable space is available.  The scripts are
 sourced with one or more arguments, the first of which defines the hook phase:
 
-    source | stack | unstack | clean
+    override_defaults | source | stack | unstack | clean
 
-    source: always called first in any of the scripts, used to set the
-        initial defaults in a lib/* script or similar
+    override_defaults: always called first in any of the scripts, used to
+        override defaults (if need be) that are otherwise set in lib/* scripts
+
+    source: called by stack.sh. Used to set the initial defaults in a lib/*
+        script or similar
 
     stack: called by stack.sh.  There are four possible values for
         the second arg to distinguish the phase stack.sh is in: