Fix comments on _bt_skiparray_strat_increment/decrement

Hi,

The attached patch is a trivial change to fix the comments on
_bt_skiparray_strat_increment() and _bt_skiparray_strat_decrement() so
that they are consistent with the comments on _bt_skiparray_strat_adjust(). 
At least the comment on _bt_skiparray_strat_decrement() containts an
obvious typo, since it mentions converting the high_compare key instead of
the low_compare key.

Regards,
Yugo Nagata

-- 
Yugo Nagata <nagata@sraoss.co.jp>