pgsql: Add comment in ginxlog.h about block used with ginxlogInsertList

Add comment in ginxlog.h about block used with ginxlogInsertListPage

All the other structures describe the list of blocks used, and in the
case of a GIN_INSERT_LISTPAGE record block 0 refers to a list page with
the items added to it.

Author: Kirill Reshke <reshkekirill@gmail.com>
Reviewed-by: Andrey Borodin <x4mmm@yandex-team.ru>
Discussion: /messages/by-id/CALdSSPgk=9WRoXhZy5fdk+T1hiau7qbL_vn94w_L1N=gtEdbsg@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/42c6b74d8928885135ba35ffdb7fb60602953e2d

Modified Files
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src/include/access/ginxlog.h | 3 +++
1 file changed, 3 insertions(+)