document what stats.active does not track

[Jan Beich]

Based on http://jemalloc.net/mailman/jemalloc-discuss/2012-March/000164.html
---
 doc/jemalloc.xml.in | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/doc/jemalloc.xml.in b/doc/jemalloc.xml.in
index 441c1a4..f1ae576 100644
--- a/doc/jemalloc.xml.in
+++ b/doc/jemalloc.xml.in
@@ -1503,25 +1503,27 @@ malloc_conf = "xmalloc:true";]]></programlisting>
 
       <varlistentry id="stats.active">
         <term>
           <mallctl>stats.active</mallctl>
           (<type>size_t</type>)
           <literal>r-</literal>
           [<option>--enable-stats</option>]
         </term>
         <listitem><para>Total number of bytes in active pages allocated by the
         application.  This is a multiple of the page size, and greater than or
         equal to <link
         linkend="stats.allocated"><mallctl>stats.allocated</mallctl></link>.
-        </para></listitem>
+        This does not include <link linkend="stats.arenas.i.pdirty">
+        <mallctl>stats.arenas.<i>.pdirty</mallctl></link> and pages
+        entirely devoted to allocator metadata.</para></listitem>
       </varlistentry>
 
       <varlistentry>
         <term>
           <mallctl>stats.mapped</mallctl>
           (<type>size_t</type>)
           <literal>r-</literal>
           [<option>--enable-stats</option>]
         </term>
         <listitem><para>Total number of bytes in chunks mapped on behalf of the
         application.  This is a multiple of the chunk size, and is at least as
         large as <link
@@ -1619,25 +1621,25 @@ malloc_conf = "xmalloc:true";]]></programlisting>
         arena.</para></listitem>
       </varlistentry>
 
       <varlistentry>
         <term>
           <mallctl>stats.arenas.<i>.pactive</mallctl>
           (<type>size_t</type>)
           <literal>r-</literal>
         </term>
         <listitem><para>Number of pages in active runs.</para></listitem>
       </varlistentry>
 
-      <varlistentry>
+      <varlistentry id="stats.arenas.i.pdirty">
         <term>
           <mallctl>stats.arenas.<i>.pdirty</mallctl>
           (<type>size_t</type>)
           <literal>r-</literal>
         </term>
         <listitem><para>Number of pages within unused runs that are potentially
         dirty, and for which <function>madvise<parameter>...</parameter>
         <parameter><constant>MADV_DONTNEED</constant></parameter></function> or
         similar has not been called.</para></listitem>
       </varlistentry>
 
       <varlistentry>