pgsql: Expand comment for isset_offset.

Expand comment for isset_offset.

This field was added in commit 0164a0f9ee to provide a way to
determine whether a storage parameter was explicitly set for the
relation or if it just picked up the default value. In most cases,
this can be accomplished by giving the storage parameter a special
out-of-range default value (e.g., the
autovacuum_vacuum_insert_threshold storage parameter defaults to
-2), but this approach doesn't work in all cases. For example, a
Boolean storage parameter cannot be given an out-of-range default,
so we need another way to discover the source of its value.

Reported-by: "David G. Johnston" <david(dot)g(dot)johnston(at)gmail(dot)com>
Reviewed-by: "David G. Johnston" <david(dot)g(dot)johnston(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAKFQuwYKtEUYKS%2B18gRs-xPhn0qOJgM2KGyyWVCODHuVn9F-XQ%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/7d559c8580f722dc40f2c34573e53e33626679e0

Modified Files
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src/include/access/reloptions.h | 14 +++++++++++++-
1 file changed, 13 insertions(+), 1 deletion(-)