Fix Gin README.

The README incorrectly claimed that GIN posting tree pages contain an array
of uncompressed items in addition to compressed posting lists. Earlier
versions of the GIN posting list compression patch worked that way, but not
the one that was committed.
Fix bug in the new B-tree incomplete-split code.

Forgot to update LSN of left sibling's page, when creating a new root.
I fixed this for regular insertions and page splits earlier, but missed
new root creation.