pgsql: Adjust OS-specific kernel settings to mention old and new BSD

Log Message:
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Adjust OS-specific kernel settings to mention old and new BSD methods of
adjusting values:

But to be on the safe side, it would make sense to do something similar
to the BSD section, and comment about older distributions maybe needing
to manipulate /proc/kernel/* directly.

Mark Kirkwood

Modified Files:
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pgsql/doc/src/sgml:
runtime.sgml (r1.304 -> r1.305)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/runtime.sgml.diff?r1=1.304&r2=1.305)