Fix documentation bug: TSOs are *not* unconditionally kept on the mutable list.

The bug where TSOs were unconditionally kept on the mutable list was #1589
which was fixed in 04cddd339c000df6d02c90ce59dbffa58d2fe166.
Curiously enough, the commit that changed this comment
0417404f5d1230c9d291ea9f73e2831121c8ec99 occurred *after* this
change was made; I can only assume Simon Marlow accidentally forgot
that he had fixed this bug. :-)

Signed-off-by: Edward Z. Yang <ezyang@mit.edu>