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

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/96bd49437cca5d7a6e4024c602a9fdbdbf7c7433

Modified Files
--------------
src/backend/replication/logical/logical.c | 5 +-
src/backend/replication/slotfuncs.c | 78 ++++++++++++++++++++-----------
2 files changed, 54 insertions(+), 29 deletions(-)