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