Adjust pg_stat_get_lock() prorows to match lock types

Hi,

I read the code of pg_stat_lock() and played a bit with it. I happened to notice one thing: the function always returns 12 rows, but the planner estimates 10 rows:

```
evantest=# EXPLAIN ANALYZE SELECT * FROM pg_catalog.pg_stat_lock;
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------
Function Scan on pg_stat_get_lock l (cost=0.00..0.10 rows=10 width=64) (actual time=0.067..0.071 rows=12.00 loops=1)
Planning Time: 0.121 ms
Execution Time: 0.126 ms
(3 rows)
```

Then I found that, in pg_proc.dat, the function's prorows is defined as 10. Since the function returns one row per lock type, and lock types are not something that change frequently, I think it is better to give the planner a more accurate row count. After changing prorows to 12, the plan looks like this:

```
evantest=# EXPLAIN ANALYZE SELECT * FROM pg_catalog.pg_stat_lock;
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------
Function Scan on pg_stat_get_lock l (cost=0.00..0.12 rows=12 width=64) (actual time=0.134..0.138 rows=12.00 loops=1)
Planning:
Buffers: shared hit=13
Planning Time: 0.313 ms
Execution Time: 0.228 ms
(5 rows)
```

While there, I also made two small tweaks to two function comments in pgstat_lock.c. If those are not considered worth changing, I am okay with removing them from the patch.

Please see the attached patch for details.

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/