[4.0] e4c66f7 Move purging into the performance section and explain its role

[Per Buer]

commit e4c66f75b38930fd0e87ee2eecf23da1339f26fe
Author: Per Buer <perbu at varnish-software.com>
Date:   Thu Feb 6 14:26:38 2014 +0100

    Move purging into the performance section and explain its role

diff --git a/doc/sphinx/users-guide/operation.rst b/doc/sphinx/users-guide/operation.rst
index dc45e7e..ed6f5e0 100644
--- a/doc/sphinx/users-guide/operation.rst
+++ b/doc/sphinx/users-guide/operation.rst
@@ -6,7 +6,6 @@ XXX: These are chapters which need to find a new home in the other sections.
 .. toctree::
    :maxdepth: 2
 
-   operation-cli
    purging
    compression
    esi
diff --git a/doc/sphinx/users-guide/performance.rst b/doc/sphinx/users-guide/performance.rst
index c28dea0..1dbb991 100644
--- a/doc/sphinx/users-guide/performance.rst
+++ b/doc/sphinx/users-guide/performance.rst
@@ -6,7 +6,16 @@ Varnish and Website Performance
 This section is about tuning the performance of your Varnish server,
 and about tuning the performance of your website using Varnish.
 
+The section is split into two. One deals with the various tools and
+functions of Varnish that you should be aware of and the other focuses
+on the how to purge content out of your cache. Purging of content is
+essential in a performance context because it allows you to extend the
+*time-to-live* (TTL) of your cached objects. Having a long TTL allows
+Varnish to keep the content in cache longer, meaning Varnish will make
+send fewer requests to your relativly slow backend.
+
 .. toctree::
    :maxdepth: 2
 
    increasing-your-hitrate
+   purging