Re: pg_get_multixact_members not documented

> > Want to put together a patch?
>
> Yes, will do

v1-0001 is the documentation only patch. I improved upon the description
suggested in [0]

> > For extra credit, maybe we could add a test or two, too...

I can add tests, even though we don't really have system function specific
testing.

A simple test will be a regress/sql which ensure the XID and lock mode
of a transaction using a savepoint, something like the below. To do anything
fancier with concurrency, we will need an isolation test.

```
drop table if exists t;
create table t (v int); insert into t values (1);
begin;
select from t for update ;
savepoint s1;
update t set v = v;
select pg_get_multixact_members(a.relminmxid), a.relminmxid from
(select relminmxid from pg_class where relname = 't') a;
commit;
```

Thoughts?

--
Sami Imseih
Amazon Web Services (AWS)

[0] https://www.postgresql.org/message-id/20150619215231.GT133018@postgresql.org