doc: BRIN indexes and autosummarize

Here's a patch to clarify the BRIN indexes documentation, particularly with
regards
to autosummarize, vacuum and autovacuum. It basically breaks down a big
blob of a
paragraph into multiple paragraphs for clarity, plus explicitly tells how
summarization
happens manually or automatically.

I also added cross-references to various relevant sections, including the
create index
page.

On this topic... I'm not familiar with with the internals of BRIN indexes
and in
backend/access/common/reloptions.c I see:

        {
            "autosummarize",
            "Enables automatic summarization on this BRIN index",
            RELOPT_KIND_BRIN,
            AccessExclusiveLock
        },

Is the exclusive lock on the index why autosummarize is off by default?

What would be the downside (if any) of having autosummarize=on by default?

Roberto

--
Crunchy Data - passion for open source PostgreSQL