BugFix: Add documentation for 'read as' capability

An earlier change that added the 'read as' capability was missing
documentation.

Change-Id: Id9d75621599177997728df4594f46a5e61f3c2c3
diff --git a/Documentation/access-control.txt b/Documentation/access-control.txt
index d3f5d77..05c2731 100644
--- a/Documentation/access-control.txt
+++ b/Documentation/access-control.txt
@@ -1356,6 +1356,12 @@
 command, but also to the web UI results pagination size.
 
 
+[[capability_readAs]]
+=== Read As
+
+Allow users to impersonate any user to see which refs they can read.
+
+
 [[capability_runAs]]
 === Run As