[DOC] Add detail regarding resource consumption wrt max_connections

The documentation for max_connections does not mention that just by having
a higher value for max_connections, PostgreSQL will use more resources.

While working with different customers, I noticed that several of them set
max_connections to very high numbers, even though they never expected to
actually have that many connections to their PostgreSQL instance.

In one extreme case, the user set max_connections to 200000 and was
befuddled that the instance was using more memory than another with the
same number of connections.

This patch adds language to the documentation pointing to the fact that
higher value of max_connections leads to higher consumption of resources by
Postgres, adding one paragraph to doc/src/sgml/config.sgml

<para>
PostgreSQL sizes certain resources based directly on the value of
<varname>max_connections</varname>. Increasing its value leads to
higher allocation of those resources, including shared memory.
</para>

Sincerely,

Roberto Mello