pgsql: Add comments explaining overflow entries in the replication lag

Add comments explaining overflow entries in the replication lag tracker.

Commit 883a95646a8 introduced overflow entries in the replication lag tracker
to fix an issue where lag columns in pg_stat_replication could stall when
the replay LSN stopped advancing.

This commit adds comments clarifying the purpose and behavior of overflow
entries to improve code readability and understanding.

Since commit 883a95646a8 was recently applied and backpatched to all
supported branches, this follow-up commit is also backpatched accordingly.

Author: Xuneng Zhou <xunengzhou(at)gmail(dot)com>
Reviewed-by: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Discussion: https://postgr.es/m/CABPTF7VxqQA_DePxyZ7Y8V+ErYyXkmwJ1P6NC+YC+cvxMipWKw@mail.gmail.com
Backpatch-through: 13

Branch
------
REL_16_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/4d707f2fd72857016400ad0a7b3e3ffa4bb2d730

Modified Files
--------------
src/backend/replication/walsender.c | 13 +++++++++++++
1 file changed, 13 insertions(+)