Fix comments about deduplication updating page.

Fix comments about deduplication updating page.

nbtree deduplication passes add tuples from the original/target page to
a temp page, merging as necessary.  The temp page is copied back to the
target permanent page in the critical section.  This is similar to the
approach taken by nbtree page splits.

Adjust comments that referred to updating the original page in-place as
tuples were merged.  These were left over from earlier versions of the
deduplication patch that didn't yet use a temp page.