Add more docs for pg_surgery?

Hi, hackers

heap_force_kill/heap_force_freeze doesn’t consider other transactions that are using the same tuples even with tuple-locks.
The functions may break transaction semantic, ex:

session1
```
create table htab(id int);
insert into htab values (100), (200), (300), (400), (500);
```

session2
```
begin isolation level repeatable read;
select * from htab for share;
 id
-----
 100
 200
 300
 400
 500
(5 rows)
```

session1
```
select heap_force_kill('htab'::regclass, ARRAY['(0, 1)']::tid[]);
 heap_force_kill
-----------------

(1 row)
```

session2
```
select * from htab for share;
 id
-----
 200
 300
 400
 500
(4 rows)
```

session2 should get the same results as it's repeatable read isolation level.

By reading the doc:
```
The pg_surgery module provides various functions to perform surgery on a damaged relation. These functions are unsafe by design and using them may corrupt (or further corrupt) your database. For example, these functions can easily be used to make a table inconsistent with its own indexes, to cause UNIQUE or FOREIGN KEY constraint violations, or even to make tuples visible which, when read, will cause a database server crash. They should be used with great caution and only as a last resort.

```
I know they are powerful tools, but also a little surprise with the above example.

Should we add more docs to tell the users that the tool will change the tuples anyway even there are tuple-locks on them?

Regards,
Zhang Mingli