Rewrite comments in replication slot advance implementation

Alvaro Herrera <alvherre@alvh.no-ip.org>

Commit: 96bd49437cca5d7a6e4024c602a9fdbdbf7c7433
Author: Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: 2018-07-19T18:15:44Z
Releases: 11.0
Rewrite comments in replication slot advance implementation

The code added by 9c7d06d60680 was a bit obscure; clarify that by
rewriting the comments.  Lack of clarity has already caused bugs, so
it's a worthy goal.

Co-authored-by: Arseny Sher <a.sher@postgrespro.ru>
Co-authored-by: Michaël Paquier <michael@paquier.xyz>
Co-authored-by: Álvaro Herrera <alvherre@alvh.no-ip.org>
Reviewed-by: Petr Jelínek <petr.jelinek@2ndquadrant.com>
Discussion: https://postgr.es/m/87y3fgoyrn.fsf@ars-thinkpad

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