Improve wording in a few comments

Improve wording in a few comments

Initially this was to fix the "catched" typo, but I (David) wasn't quite
clear on what the previous comment meant about being "effective".  I
expect this means efficiency, so I've reworded the comment to indicate
that.

While this is only a comment fixup, for the sake of possibly minimizing
possible future backpatching pain, I've opted to backpatch to 18 since
this code is new to that version and the release isn't out the door yet.

Author: Tender Wang <tndrwang@gmail.com>
Discussion: https://postgr.es/m/CAHewXNmSYWPud1sfBvpKbCJeRkWeZYuqatxtV9U9LvAFXBEiBw@mail.gmail.com
Backpatch-through: 18