pgsql: Remove paragraph about Linux OOM killer and fork().

Log Message:
-----------
Remove paragraph about Linux OOM killer and fork(). Instead link to
article about OOM.

Modified Files:
--------------
pgsql/doc/src/sgml:
runtime.sgml (r1.392 -> r1.393)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/runtime.sgml?r1=1.392&r2=1.393)