Fix comments atop ReorderBufferAddInvalidations.
The comments atop seem to indicate that we always accumulate invalidation
messages in a top-level transaction which is neither required nor matches
with the code.
Author: Amit Kapila
Reviewd by: Masahiko Sawada
Backpatch-through: 14, where it was introduced in commit
c55040ccd0
Discussion: https://postgr.es/m/CAA4eK1LxGgnUroPz8STb6OfjVU1yaHoSA+T63URwmGCLdMJ0LA@mail.gmail.com