Allow opt.tcache_max to accept small size classes.

Previously all the small size classes were cached.  However this has downsides
-- particularly when page size is greater than 4K (e.g. iOS), which will result
in much higher SMALL_MAXCLASS.

This change allows tcache_max to be set to lower values, to better control
resources taken by tcache.

doc/jemalloc.xml.in

@@ -1313,7 +1313,7 @@ malloc_conf = "xmalloc:true";]]></programlisting>
           <literal>r-</literal>
         </term>
         <listitem><para>Maximum size class to cache in the thread-specific cache
-        (tcache).  At a minimum, all small size classes are cached; and at a
+        (tcache).  At a minimum, the first size class is cached; and at a
         maximum, size classes up to 8 MiB can be cached.  The default maximum is
         32 KiB (2^15).  As a convenience, this may also be set by specifying
         lg_tcache_max, which will be taken to be the base-2 logarithm of the