Fix the description of GUC "max_locks_per_transaction" and "max_pred_locks_per_transaction" in guc_table.c

Hi,

When I refer to the GUC "max_locks_per_transaction", I find that the description
of the shared lock table size in pg-doc[1] is inconsistent with the code
(guc_table.c). BTW, the GUC "max_predicate_locks_per_xact" has similar problems.

I think the descriptions in pg-doc are correct.
- GUC "max_locks_per_transaction"
Please refer to the macro "NLOCKENTS" used to obtain max_table_size in the
function InitLocks.

- GUC "max_predicate_locks_per_xact"
Please refer to the macro "NPREDICATELOCKTARGETENTS" used to obtain
max_table_size in the function InitPredicateLocks.

Attach the patch to fix the descriptions of these two GUCs in guc_table.c.

[1] - https://www.postgresql.org/docs/devel/runtime-config-locks.html

Regards,
Wang Wei