Fix pg_buffercache document

Hi,

The following description in pg_buffercace is no longer true.

When the pg_buffercache view is accessed, internal buffer manager
locks are taken for long enough to copy all the buffer state data that
the view will display. This ensures that the view produces a
consistent set of results, while not blocking normal buffer activity
longer than necessary. Nonetheless there could be some impact on
database performance if this view is read often.

We changed pg_buffercache_page so that it doesn't take buffer manager
locks in commit 6e654546fb6. Therefore from version 10,
pg_buffercache_page has less impact on normal buffer activity less but
might not return a consistent snapshot across all buffers instead.

I've attached a patch to fix the documentation.

Regards,

--
Masahiko Sawada http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services