document what stats.active does not track

[Jason Evans]

On Oct 17, 2012, at 1:06 PM, Jan Beich wrote:
> Based on http://jemalloc.net/mailman/jemalloc-discuss/2012-March/000164.html
> ---
> doc/jemalloc.xml.in | 6 ++++--
> 1 file changed, 4 insertions(+), 2 deletions(-)

Applied.

Thanks,
Jason