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: https://postgr.es/m/CALdSSPgk=9WRoXhZy5fdk+T1hiau7qbL_vn94w_L1N=gtEdbsg@mail.gmail.com

src/include/access/ginxlog.h

@@ -179,6 +179,9 @@ typedef struct ginxlogUpdateMeta
 
 #define XLOG_GIN_INSERT_LISTPAGE  0x70
 
+/*
+ * Backup Blk 0: list page with inserted tuples
+ */
 typedef struct ginxlogInsertListPage
 {
 	BlockNumber rightlink;