pgsql: Improve description of some WAL records for GIN

Improve description of some WAL records for GIN

The following information is added in the description of some GIN
records:
- In INSERT_LISTPAGE, the number of tuples and the right link block.
- In UPDATE_META_PAGE, the number of tuples, the previous tail block,
and the right link block.
- In SPLIT, the left and right children blocks.

Author: Kirill Reshke <reshkekirill(at)gmail(dot)com>
Reviewed-by: Michael Paquier <michael(at)paquier(dot)xyz>
Reviewed-by: Andrey Borodin <x4mmm(at)yandex-team(dot)ru>
Discussion: https://postgr.es/m/CALdSSPgnAt5L=D_xGXRXLYO5FK1H31_eYEESxdU1n-r4g+6GqA@mail.gmail.com

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/138da727a174219da2d408382e50f8628f1fa38f

Modified Files
--------------
src/backend/access/rmgrdesc/gindesc.c | 24 ++++++++++++++++++++++--
1 file changed, 22 insertions(+), 2 deletions(-)