Tweak some nbtree-related code comments.

Peter Geoghegan <pg@bowt.ie>

Commit: 9c7fb7e6d8d0fdcabbdc32daad2159812e538bd5
Author: Peter Geoghegan <pg@bowt.ie>
Date: 2019-03-29T19:29:05Z
Releases: 12.0
Tweak some nbtree-related code comments.

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