Add a hard limit on tcache max size class.

For locality reasons, tcache bins are integrated in TSD.  Allowing all size
classes to be cached has little benefit, but takes up much thread local storage.
In addition, it complicates the layout which we try hard to optimize.

doc/jemalloc.xml.in

@@ -1314,8 +1314,8 @@ malloc_conf = "xmalloc:true";]]></programlisting>
         </term>
         <listitem><para>Maximum size class (log base 2) to cache in the
         thread-specific cache (tcache).  At a minimum, all small size classes
-        are cached, and at a maximum all large size classes are cached.  The
-        default maximum is 32 KiB (2^15).</para></listitem>
+        are cached; and at a maximum, size classes up to 8 MiB can be cached.
+        The default maximum is 32 KiB (2^15).</para></listitem>
       </varlistentry>
 
       <varlistentry id="opt.thp">