Improve comments about btree's use of ScanKey data structures: there

Improve comments about btree's use of ScanKey data structures: there
are two basically different kinds of scankeys, and we ought to try harder
to indicate which is used in each place in the code.  I've chosen the names
"search scankey" and "insertion scankey", though you could make about
as good an argument for "operator scankey" and "comparison function
scankey".