Document nbtree row comparison design.

Document nbtree row comparison design.

Add comments explaining when and where it is safe for nbtree to treat
row compare keys as if they were simple scalar inequality keys on the
row's most significant column.  This is particularly important within
_bt_advance_array_keys, which deals with required inequality keys in a
general and uniform way, without any special handling for row compares.

Also spell out the implications of _bt_check_rowcompare's approach of
_conditionally_ evaluating lower-order row compare subkeys, particularly
when one of its lower-order subkeys might see NULL index tuple values
(these may or may not affect whether the qual as a whole is satisfied).
The behavior in this area isn't particularly intuitive, so these issues
seem worth going into.

In passing, add a few more defensive/documenting row comparison related
assertions to _bt_first and _bt_check_rowcompare.

Follow-up to commits bd3f59fd and ec986020.

Author: Peter Geoghegan <pg@bowt.ie>
Reviewed-By: Victor Yegorov <vyegorov@gmail.com>
Reviewed-By: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/CAH2-Wznwkak_K7pcAdv9uH8ZfNo8QO7+tHXOaCUddMeTfaCCFw@mail.gmail.com
Backpatch-through: 18