[3.0] 5e36c5e Explain what a purge is.

[Tollef Fog Heen]

commit 5e36c5e0e9bcb203e30e4ba81a34c945aa1ee79b
Author: Per Buer <perbu at varnish-software.com>
Date:   Thu Jun 30 10:18:36 2011 +0200

    Explain what a purge is.

diff --git a/doc/sphinx/tutorial/purging.rst b/doc/sphinx/tutorial/purging.rst
index 99f3f77..bc40691 100644
--- a/doc/sphinx/tutorial/purging.rst
+++ b/doc/sphinx/tutorial/purging.rst
@@ -17,6 +17,10 @@ bans. First, let me explain the HTTP purges.
 HTTP Purges
 ===========
 
+A *purge* is what happens when you pick out an object from the cache
+and discard it along with its variants. Usually a purge is invoked
+through HTTP with the method PURGE.
+
 An HTTP purge is similar to an HTTP GET request, except that the
 *method* is PURGE. Actually you can call the method whatever you'd
 like, but most people refer to this as purging. Squid supports the