pgsql: Doc: clarify description of CREATE/ALTER FUNCTION ... SET FROM C

Doc: clarify description of CREATE/ALTER FUNCTION ... SET FROM CURRENT.

Per discussion with David Johnston.

Branch
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REL9_2_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/be1d75ea23ee020d0c5e27195a01d7678aa1b5ef

Modified Files
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doc/src/sgml/ref/alter_function.sgml  | 5 +++--
doc/src/sgml/ref/create_function.sgml | 5 +++--
2 files changed, 6 insertions(+), 4 deletions(-)