Add code comment explaining ins_since_vacuum and aborted inserts

Sami complained that there's a discrepancy between n_mod_since_analyze
and n_ins_since_vacuum, as the former only accounts for committed changes
and the latter tracks committed and aborted inserts.  Nobody seemed
overly concerned that this would cause any concerning issues.  The
repercussions, from what I can tell, are limited to causing an
autovacuum to trigger for inserts sooner than it otherwise might. For
typical ratios of commits to aborts, it's unlikely to ever be noticed.

Fixing things to make it so n_ins_since_vacuum only displays committed
inserts would require an additional field in PgStat_TableCounts, which
does not quite seem worthwhile at this stage.  This commit just adds a
comment with some details to mention that we know about it, which will
hopefully prevent repeat discussions.

Reported-by: Sami Imseih <samimseih@gmail.com>
Author: David Rowley <drowleyml@gmail.com>
Reviewed-by: Sami Imseih <samimseih@gmail.com>
Discussion: https://postgr.es/m/CAApHDvpgV3a-R2EGmPOh0L-x3pHbZpM3y4dySWfy+UqUazwDQA@mail.gmail.com

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