Document the order of the X-Cache header

Author: mlibbey

doc/admin-guide/plugins/xdebug.en.rst

@@ -78,7 +78,7 @@ X-Cache-Key
     key is being used.
 
 X-Cache
-    The ``X-Cache`` header contains the results of any cache lookup.
+    The ``X-Cache`` header contains the results of any cache lookups.
 
     ==========  ===========
     Value       Description
@@ -90,6 +90,10 @@ X-Cache
     skipped     The cache lookup was skipped.
     ==========  ===========
 
+    If a request goes through multiple proxies, each one prepends its X-Cache header content
+    at the beginning of the existing X-Cache header. As a result, the order is reversed from
+    the Via: header.
+
 X-Cache-Generation
   The cache generation ID for this transaction, as specified by the
   :ts:cv:`proxy.config.http.cache.generation` configuration variable.