pgsql: Rewrite comments in replication slot advance implementation

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/1573995f55994ee04dd0d69481de17d662ad8e88

Modified Files
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src/backend/replication/logical/logical.c |  5 ++-
src/backend/replication/slotfuncs.c       | 72 ++++++++++++++++++++-----------
2 files changed, 50 insertions(+), 27 deletions(-)