pgsql: Clarify STABLE function documentation to highlight how such

Log Message:
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Clarify STABLE function documentation to highlight how such functions
can be optimized.

Tags:
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REL8_1_STABLE

Modified Files:
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pgsql/doc/src/sgml:
xfunc.sgml (r1.107 -> r1.107.2.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/xfunc.sgml.diff?r1=1.107&r2=1.107.2.1)